IMPROVEMENTS IN/OR RELATING TO SLIDING PANELS FIELD OF INVENTION The present invention relates to sliding panel assemblies for buildings and in particular although not solely to provide sliding doors and windows with a reduced sill construction. BACKGROUND Conventional sliding doors in the New Zealand residential market today generally have the moving panels 1 sliding on bottom mounted wheels 2 running on a sill mounted track 3 in a captivated channel 4, usually with mohair type seals either side, as shown in cross section in Figure 1. A stacker door is a sliding panel unit which may for example include two (or more) moving panels in separate tracks one behind the other which allow both panels to slide and "stack" behind or in front of a third panel which is fixed, thereby creating a stack of three panels (two moving and one fixed). The benefit of a stacker door with two moving panels in two separate tracks allows a clear door opening of for example 2/3 of the total unit width as opposed to a conventional sliding door with one track and one moving panel which can only achieve 1A clear opening. A stacker door is not limited to two tracks and two moving panels. A stacker door can also be mirrored to allow two panels sliding back from the mid point of the door unit each way, making 4 sliding panels and two fixed panels (or more depending on the number of tracks). Such a design has a number of limitations, such as the sill having a lot of functional detail required for the workings of the unit. However these details create a large upwardly opening trough with ridges and crevices that attract and hold dirt, moisture and construction debris. Cleaning is also not easy with this amount of detail. A further limitation with large channel type tracking frames and bottom rolling panels is that the dirt and construction debris can reduce the life of the wheels. A common failure is fractured nylon tyres, damaged tracks, and bearing degradation as the wheels invariably end up operating in a track area that has collected debris. The danger period for a newly installed unit is usually during the building phase, where construction debris is at its highest level. Longer term, degradation of the wheels is caused by poor housekeeping, sandy locations at coastal areas, or similar environments. Such visible trough and crevice tracks also detract from the lines of the building and can be aesthetically unpleasing. It is therefore an object of the present invention to provide a sliding panel assembly which has the ability to provide a reduced sill construction and/or which goes at least someway to overcoming the disadvantages of the prior art, or to at least provide the public with a useful choice. BRIEF DESCRIPTION OF THE INVENTION Accordingly in a first aspect the present invention may be said to broadly consist in a sliding door or window assembly, the assembly including or comprising a frame of at least a head frame member and a sill frame member to provide an at least in part frame defined opening, and at least on sliding panel being (a) mounted by load bearing elements (e.g. wheels) at least two points to said head frame member, and (b) engaged in sliding engagement with said sill frame member via a sill running track extending at least in part along said sill frame member and that is defined between sill upper and lower regions at an undercut, wherein said sliding panel includes at least one bottom runner engaged with said running track, which allows guided sliding of said sliding panel parallel to said at least in part frame defined opening but which substantially prevents movement perpendicular to the plane of the opening. Preferably said sliding panel lies outwards of the plane of the opening and suspended above said sill frame member. Preferably said sill frame lower region is at least in part exposed outwardly by said sliding panel, and is non perpendicular to the plane of the opening. Preferably said sill frame lower region is at least in part exposed outwardly by said sliding panel, and is sloped downwards away from said undercut. Preferably said sill frame member provides undercut with an downward facing opening on said sill frame member and said opening is substantially parallel to the plane of said opening. Preferably there is a friction reducing material on either said bottom ranner(s) or said running track. Preferably said friction reducing material is located on said running truck. Preferably said friction reducing material is nylon. Preferably said bottom runner(s) is/are slidingly engaged via said outward opening with said running track. Preferably there is adjustment means in the vertical plane parallel to the plane of the opening, of the angle of the sliding panel relative to said head frame and sill frame members to account for any relative misalignment. Preferably said adjustment means is provided by variable height mechanism the wheeled engagement from said head frame member to the top of said moving panel. Preferably the first of said two points is at or towards a leading stile of said sliding panel ("leading stile wheel") and the second of said two points is at or towards a trailing stile of said sliding panel ("trailing wheel stile"). Preferably only one of the load bearing elements has such adjustment. Preferably only the leading stile load bearing element of said sliding panel has such adjustment. Preferably said load bearing elements are wheeled engagements with a track within said head frame member. Preferably said undercut is inwards of the outer most edge of the sill frame member and upwards from the lower most edge of the sill frame member and said bottom runner is mounted to said sill frame member at said undercut. Preferably there are two said bottom runners, one at either stile end of said sliding panel. In a second aspect the present invention consists in a window and/or door assembly of or for a building, said assembly comprising or including at - A - least one or more window and/or door sliding panel(s) (glazed or otherwise) adapted, to lie parallel in the plane (hereafter "said plane") of a wall opening (at least in part) thereby, wherein said sliding panel is tracked by tracking means comprising or including a) load bearing elements disposed at at least two points to engage a track of said head frame member, and b) runners disposed at a lower extremity (or thereabouts) of said sliding panel to engage said sill frame member at a guide track of said sill frame member which is exposed to a direction not directly accessible above said sill. In a third aspect of the present invention a sliding door or window assembly, the assembly comprising or including, at least one first sliding panel, and a head frame member defining at least one upper track said sliding panel mounted by upper load bearing elements (e.g. wheels) at at least two points to said upper track, a sill frame member defining at least one lower running track said at least one sliding panel engaged in sliding engagement with said sill frame member via said lower running track that extends at least in part along, and is located at an undercut region of, the sill frame member, wherein at least one bottom runner of said at least one sliding panel follows in said lower running track, which allows sliding of said at least one sliding panel relative to the at least in part frame defined opening but which substantially prevents movement out of the plane of the opening, of said at least one sliding panel. Preferably said at least one sliding panel lies outwards of the plane of the opening and is suspended above sill frame member. Preferably where there is at first of said at least one sliding panel, and subsequent and successive of said sliding panel each lying outward from said first sliding panel and each to run in subsequent and successive outward more from the first mentioned upper track in said head frame member and subsequent and successive lower tracks in said sill frame member so that said subsequent and successive sliding panels mount parallel to said at least one sliding panel, yet are spaced perpendicularly outward from the plane of said opening. Preferably said undercut at which the at least the first of said running track(s) is located is between upper and lower regions of said sill frame member. Preferably said sill frame member lower region is at least in part exposed outwardly by said sliding panel, and is non perpendicular to the plane of the opening. Preferably said sill frame member lower region is at least in part exposed outwardly by said sliding panel, and is sloped downwards away from said undercut. Preferably said undercut has an outward opening on said sill frame member and said opening is substantially parallel to the plane of said opening. Preferably there is a friction reducing material on either said bottom runner(s) or said running track. Preferably said friction reducing material is located on said running track. Preferably said friction reducing material is nylon. Preferably said bottom runner(s) is/are slidingly engaged via said outward opening in said at least one running track for the full range of sliding of said at least one sliding panel. Preferably there is adjustment means in the vertical plane of the angle of said at least one sliding panel relative to said head frame member and sill frame members to account for any relative misalignment. Preferably said adjustment is by variance of the length of the load bearing elements from said head frame member to the top of said at least one sliding panel. Preferably the first of said at least two points is at or towards the leading stile of said at least one sliding panel and the second of said two points is at or towards the trailing stile of said at least one sliding panel. Preferably only one of said load bearing elements has such adjustment. Preferably only the leading stile load bearing element of said at least one sliding panel has such adjustment. Preferably said load bearing elements are wheeled engagements with a track within said head frame member. Preferably said undercut is inwards and upwards of the sill frame and said bottom runner tucks into said undercut. Preferably there are two said bottom runners, one at either stile end of each of said at least one sliding panel. Preferably there is a continuous seal presented to the lower part of said at least one sliding panel(s) by or from the sill frame member, when said at least one sliding panel is in a closed position. Preferably said continuous seal is a mohair type seal. Preferably where there are two or more sliding panels present, spaced perpendicularly from the plane of said opening relative to each other, said continuous seal is presented so as to be continuous against said lower part of all said sliding panels when in said closed condition. Preferably said continuous seal bears against the inner face of said lower part of said at least one sliding panel. Preferably said continuous seal, for each subsequent and successive sliding panel outward more from said at least one sliding panel, is stepped outward to bear against the inner face of said lower part of said each subsequent and successive sliding panel. Preferably said outward stepping of said continuous seal is by a spacing member. Preferably said spacing member in addition to carrying part of said continuous seal provides a bump stop for the sliding panel inward to that outward sliding panel it steps outward to. In a fourth aspect the present invention consist of an interconnection member ("spacing member") for a sliding panel assembly, comprising or including, a seal retaining means for retaining a seal, wherein said interconnection member is located between a first seal bearing surface of a first frame member and a second seal bearing surface of a second frame member, spaced outward from said first seal bearing surface, to retain and allow continuous running of said seal, via said interconnection member from said first seal bearing surface to said second seal bearing surface, and an engagement means for engaging with an aperture in anyone of said first and second frame members. Preferably said retaining means is formed from an upper rebate and lower rebate in said interconnecting member, forming a "T" shaped channel slot therein. Preferably said member is or a two part construction, having an upper part and a lower part, separated therebetween. Preferably said upper part has said upper rebate and said lower part has said lower rebate therein for capturing and retaining of said seal when said two parts are brought together. Preferably where said first and second frame members are sill or sill providing members of a sliding panel assembly. Preferably said second frame member is a sill providing member for an outer sliding panel and said first frame member is a sill providing member for an inner sliding panel. In a fifth aspect the present invention consists in a building with a sliding panel assembly as herein before described. In a sixth aspect the present invention consists in a building with a sliding panel assembly as herein described with reference to any one or more of Figures 2 to 10. In a seventh aspect the present invention consists in a sill member with an upper surface and a lower sloped surface between which there is a running track providing undercut running the length of the sill member. In a eighth aspect the present invention consists in a sliding panel assembly as herein described with reference to any one or more of Figures 2 to 11 or Figures 13 to 30. In a ninth aspect the present invention consists of a building with a sliding door or window assembly as herein described with reference to any one or more of Figures 2 to 11 or Figures 13 to 30. In a tenth aspect of the present invention consists of a sliding panel assembly as herein described with reference to any one or more of Figures 2 to 11 or Figures 13 to 30. In an eleventh aspect of the present invention consists of an interconnection or space member as herein described with reference to any one or more of Figures 2 to 11 or Figures 13 to 30. In a twelfth aspect of the present invention consist of an interconnection or space member as herein described with reference to any one or more of Figures 2 to 11 or Figures 13 to 30. Wherein herein the word door is used this is taken to include any sliding panel, such as those for a door or window, whether glazed or otherwise. To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. The term 'comprising' as used in this specification and claims means 'consisting at least in part of, that is to say when interpreting independent claims including that term, the features prefaced by that term in each claim will need to be present but other features can also be present. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be discussed with reference to the accompanying drawings in which; Figure 1 is a vertical cross-section of a panel (as a sliding door) of the prior art showing the lower rail engagement system,

Figure 2 is a series of horizontal projection views of various embodiments which may use a sliding panel of the present invention, in these embodiments the sliding panel is a sliding door, but may equally be a window or similar, and also is shown a building with the sliding panel assembly installed therein

Figure 3 is a side elevation of the sliding component of a sliding panel assembly including a revolved cross-section BB being the cross-section of the lower rail together with bottom runner mounted therein, Figure 4A is a plan view of the leading stile seen from A in Figure 3 showing the leading wheel set and leading stile and Figure 4B is a plan view of the trailing stile seen from B in Figure 3 showing the trailing wheel set and trailing stile, Figure 5 is an end view of the trailing end of the sliding panel including a revolved section CC at cross section CC, Figure 6 is a similar view to Figure 5 with an end on elevation of the leading edge of the sliding panel with a revolved section AA along line AA, Figure 7 is a plan view of the sliding panel and surrounding framework comprising the receiving frame (jamb frame) and the opened door way or window frame, the sliding panel being in a retracted or open position, Figure 8 is a similar view to Figure 7 where the sliding panel has been slid to a closed position to close off the doorway or opening, Figure 9 is a contracted vertical section along line DD of Figure 8 showing the wheel set in the track of the head frame and the bottom runner in the integrated track of the sill frame, Figure 10 is a similar view to that of Figure 9 except along line EE of Figure 8 with the exception that the sliding panel, wheel sets and bottom runner have been removed to show the head and sill frames, their tracks and the side light detail, and Figure 11 shows in isometric an adjustable wheel set at A, B shows the non adjustable wheel set, C shows the bottom runner for engagement with the integrated track, D is a comer soaker, E is the one form of the door stop and F is an end cap. Figure 12 shows a stacker door system of the prior art where A is with the panels in place and B is with the panels removed, clearly showing the track 3 where materials and debris can gather, the view being a vertical cross section through the sill member, Figure 13 is a vertical cross section through the head frame member of the current invention showing two tracks one each for a sliding panel, Figure 14 shows a sill frame member in vertical cross section showing a first track for a first panel and a second track for a second panel, Figure 15 shows a similar view of that of Figure 14 with a side light installed, Figure 16 shows a horizontal cross section through a jamb frame member, the jamb forming the sides of the frame of the sliding panel system, Figure 17 shows in vertical cross section a similar view to that of Figure 15 with the addition of an inner and outer sliding panel showing the engagement of each with each of their tracks, the inner sliding panel having the bottom runner partially engaged with its integrated track, Figure 18 shows a similar view to that of Figure 17 whereby both sliding panels are completely engaged with each of their respective tracks, Figure 19 shows the detail of the sill frame member at a location where only the outer sliding panel is located (when in a closed position) and therefore there is a filler extrusion present to space the outer sliding panel out from the sill frame member, Figure 20 shows a plan view with the head frame member removed, showing each of the sliding panels and wheel sets, jamb members, side light and frame, opening and sill frame member, Figure 21 is a similar view to that of Figure 20 taking through a horizontal cross section at or near the lower portion of the sliding panels, Figure 22 is a similar view to that of Figure 21 showing the wheel set removal slot location in the head frame member with the door stops to allow/prevent the removal of the wheel set from the head frame and hence the sliding panel, Figure 23 shows the adjustable single wheel set of Figure 1 IA in plan view at A side elevation B and end elevation C, Figure 24 shows the non-adjustable single wheel set of Figure 1 IB in plan view at A side elevation B and end elevation C5 Figure 25 shows in bottom isometric view the outer sliding panel engagement with the sill member, Figure 26 shows in isometric the sill frame member with the filler extrusion and zed seal bracket, Figure 27 shows in close up isometric the bottom runner of Figure 11C, Figure 28 shows in isometric the outer sliding panel bottom runner, Figure 29 is a similar view to that of Figure 26 where the end fitting has been retained by threaded fasteners, and Figure 30 shows a similar view to that of Figure 19 with the addition of the end fitting. DETAILED DESCRIPTION OF THE INVENTION SINGLE PANEL EMBODIMENT With reference to Figures 2 - 11 there is shown various views a sliding panel of the present invention. This particular embodiment shown and described may be for a door in a doorway, however this can equally be for a window also, or other opening that requires closing, regardless of whether the sliding panel is glazed or otherwise. The sliding panel assembly 84 can be installed or fabricated in a building 83. The invention provides a means of reducing at least the visible detail in the sill section of a sliding panel. Particularly the invention addresses the detail at the captivated channel 4 of the prior art example shown in Figure 1, within which the sliding panel normally rolls. The invention facilitates easy cleaning and reduced areas for trapping dirt and construction debris. This in turn provides long term durability for the running gear so that it is not as affected as a traditional bottom rolling sliding door during construction and in service. Similarly prior art sliding windows or panels can have the sill captivated channel fill with debris or materials (e.g. construction materials, or for example sand if near a beach, or dust and debris) again leading to premature wear and failure of the bottom rolling wheels. With reference to the present invention the opening 45 may be defined, at least in part by a door frame 47. The door frame 47 can consist of at least the head frame member 15, and the sill frame member 17. In the preferred embodiments the head and sill frame 15 and 17 are preferably joined at the vertices by jamb frame members 18. The four frame members thus making the door frame 47. The head 15, sill 17 and jamb 16 frames members are different. The head frame member 15 incorporates an integrated track 32. The jamb frame members 16 can incorporate channels 41, 42 at either end of the door frame 47 for the stiles 22 and 21 of the sliding panel 7 to close into. The leading panel stile 21 may have the leading jamb channel 42 to engage in when the sliding panel 7 is in the closed position (as per Figure 8). Similarly the trailing panel stile 22 may have the trailing jamb channel to engage when the sliding panel 7 is in the open position as per Figure 7. The sill frame 17 is of a low profile shape. The invention provides as clean from detail a sill surface 8 as possible. Such joinery usually needs an up stand 69 of sorts to provide for weather tightness. An up stand typically being a change in height from the outer surface to the inner surface to prevent water and debris etc from entering. The design of this sliding panel assembly has two smooth surfaces to the sill. The sill top surface 6 is flat and is on the internal dry area directly behind the sliding panel 7 and an external surface which is non-perpendicular to the plane of the opening. In one embodiment this surface is a two degree sloped surface 8 (though any angle required will suffice) under the sliding panel 7 allowing for some runoff of water.

There is an undercut 9 between these two surfaces to allow for a running track 10 for the bottom runners 14 (explained later). This feature is also ideal for keeping water away from the seals, 11. The undercut 9 is inwards and upwards of the outer most edge of the sloped surface 8 of the sill frame and upwards of the lower most edge of the sloped surface 8. The undercut 9 is defined by a downwardly facing surface 81, with an skirting 82 outwardly thereof. Load bearing elements are attached to the top of the sliding panel 7. These load bearing elements are wheeled in the preferred embodiment. The first is an adjustable wheel set 12, and the second is a non adjustable wheel set 13. The wheel sets 12 and 13 are located at the top two corners of the sliding panel and the bottom of the sliding panel is held in alignment to the sill by bottom runners 14 preferably at each corner of the panel that tuck under an undercut feature 9 in the sill. In one embodiment the apertures available in the lower section of the leading and trailing sections of the stiles require the bottom runners to be of differing size, as shown in Figure 11C, however this restriction is not to be considered limiting on the scope of the invention, and any size that is suitable to retain the panel is within the invention scope. The sliding panel 7 slides on the more outward side of the unit, unlike a conventional sliding door as shown in Figure 1. A better weather resistance is achievable with this convention. The fourth side, the mullion interlocker 20 has an integral glazing feature within the geometry of the section. There are other extrusions in the set that allow other configurations of panel to be made. Configurations shown in Figure 2 by way of example only include two panel 37 (one fixed as a sidelight 18 and one sliding panel 7), various three panel 38 (two fixed either side of a central sliding panel, two fixed to one side of a sliding panel etc), four panel 39 (two central sliding panels 7 moving away from the mid point of the door) and combinations of overlights 40 (opening sashes or fixed glass above the sliding panels 7 and fixed panels ), or further sidelights 18 (as many segments of fixed glass or opening sashes to the side of sliding panels as is necessary). The sliding panel7 preferably has two wheels sets, 12 and 13 at the top of the sliding panel 7. In the preferred embodiment the adjustable wheel set 12 is attached to the leading panel stile 21, the non adjustable wheel set 13 is attached to the trailing panel stile 22. However any combination of adjustable or non adjustable wheel sets at either or both ends may be used. The adjustable wheel set 12 has an adjustment mechanism 27 which allows (for example) the leading edge 50 of the sliding panel 7 to be moved up and down. In one preferred embodiment this allows for up and down movement of 4mm each way, though other ranges are considered within the scope also for different applications. With reference to Figure 3 the adjustment mechanism in one embodiment consists of an adjustment screw 28 which threadingly engages an adjustment block 29 which pivots a wheel set arm 35 about a pivot point 36. The weight of the sliding panel 7 tensions the contact between the adjustment screw 28 and the leading panel stile 21. Rotation of the adjustment screw 28 will vary the height of the sliding panel 7 in relation to the door frame 47. This feature is required to move a panel out of square to its opening to compensate for the door frame being poorly installed and out of square itself. If the door frame is installed square then no such adjustment will be necessary. The trailing wheel set 13 is fixed solidly to the top of the trailing stile 22 and has no up or down adjustment. As shown in figure 9 the bottom runners 14 in conjunction with the undercut 9 are used to retain the sliding panel 7 to the sill frame 17. There is preferably one bottom runner 14 at each bottom corner of each sliding panel 7. Without these bottom runners 14, the bottom edge of the sliding panel 7 could swing outwards away from the plane of the opening once it is disengaged from its home open position (where it normally engages one jamb frame 16 in the trailing jamb channel 41). When in the home closed position the leading stile 21 preferably engages the leading jamb channel 42 and there is engagement 43 of the mullion and trailing stile. The sliding panel 7 can not swing inwards as it is bearing on a mohair sealing strip, 11, mounted within the sill frame member 17. The bottom runners 14 in operation are not vertically loaded. They merely keep the sliding panel 7 in line with the sill frame member 17. The bottom runners 14 run in the running track 10 forward at the undercut 9. The running track 10 has an extruded nylon sleeve 1OA for the foot 31 of the bottom runner 14 to run in for smooth operation and long term durability. The friction reducing material, in this embodiment, nylon could equally be installed on the foot 31. The head frame member 15 has an integrated track 32 which the tyres of the wheel sets 12, 13 run in. Preferably the throat of the track 23 is narrower than the gauge of the tyres to capture the wheels in the integrated track 32 head frame member 15. To facilitate installation removal slot 24 is machined which allows the wheel sets to be inserted into the integrated track 32. This slot is strategically positioned within the integrated track 32 so that during the sliding panels 7 normal travel, neither set of wheel sets 12, 13 will fall into this removal slot 24 and disengage from the integrated track 32. This is achieved by having the removal slot 24 positioned between the two top wheel sets 12, 13 closer to the leading stile end 30 and the sliding panel's 7 movement restricted by a door stop 25 which is removable. During installation the trailing wheel set 13 is loose from the sliding panel 7 and is inserted into the removal slot 24 in the integrated track 32 and slid along away from the removal slot 24, out of the way. The adjustable wheel set 12 is fixed to the sliding panel 7 which prior to this stage is loose from the sliding panel. The sliding panel 7, now complete with leading wheel set 12 is presented to the door frame and the wheel set is pushed up through the removal slot 24 in the integrated track 32. The adjustable wheel set 12 is now engaged. The trailing end of the panel still not having a wheel set attached to it is loose but is then lifted up and over the fixing tag 34 of the non adjustable wheel set 13 and fixed to the non adjustable wheel set, 13. The sliding panel 7 now hangs and slides on both wheels sets which are engaged in the integrated track 32 of the head frame 15. An alternate method of attaching the sliding panel 7 is also contemplated. This alternate method utilizes a pair of slots 24 and 24A. The second slot, at the trailing wheel end of the head frame 15, will allow the non adjustable wheel set 13 to be fixed to the sliding panel 7 first, before inserting the assembly of the two wheel sets 12 and 13 and sliding panel 7 into the head. So, the sliding panel 7 with two wheels attached will be lifted up into two slots 24 and 24A spaced appropriately to receive the two wheel sets 12, 13 simultaneously. The wheel sets are similarly prevented from falling out the slots 24 and 24A by one of more door stops 25, as below. A removable door stop 25 is fitted to the integrated track 32 at the desired extreme open position of the door panel 7 at the trailing stile end. This limits the sliding panel's 7 travel for two purposes; firstly to stop the adjustable wheel set 12 from dropping into the top track slot, 24, and secondly to limit the opening of the sliding panel so that door furniture fitted to the leading stile 21 will not hit the mullion 20 of the frame unit and still allow finger clearance to grip the handle to shut the sliding panel 7. With reference to Figure 9 the bottom runners 14 provided to stop the bottom edge of the sliding panel 7 swinging outwards, are pre-fitted with the lower of the two available screw fitting holes to the leading stile 21 and trailing stile 22 prior to inserting the panel 7 into the frame. The bottom runners 14 are tilted downward so when the bottom edge of the guide is moved to the vertical position, the legs of the guides can easily clear the sill undercut 9 before being tilted back upward to their normal running position where the final screw of the two available screw holes is fitted to stop further rotation and position the bottom runners 14 in place to secure the bottom edge of the sliding panel 7 in relation to the sill frame member 17. Alternatively the bottom runners 14 can be fitted by laying them on their sides on the sloping surface 8 of the sill frame 17 and pushing them under the panel then standing them up in position for screws, 26, to pass through the leading and trailing stiles to secure them both. Sliding panel 7 removal is required for future maintenance to running gear or glass replacement. The bottom runners 14 have the top screw 26 A removed and the bottom screw 26B loosened which enables the bottom runner 14 to tilt downward at its engaging leg released from the undercut feature 9. Altematively the bottom runners 14 are unscrewed and separated from each end of the sliding panel and are tipped over and withdrawn from the sill, 17. The bottom edge of the sliding panel 7 is now free. The doorstop 25 in the integrated track is able to be removed. The trailing stile wheel set 13 is unscrewed from the trailing stile 22 and the trailing end of the sliding panel is withdrawn off the wheel set 13 which stays within the top track 15. The sliding panel 7 is free to be moved to the top track slot 24 and the sliding panel 7 with adjustable wheel 12 still fitted can be dropped out the slot. The non adjustable wheel 13 can be run along the track to the slot 24 and removed if necessary. If the alternative two slot method is used then the removal requires the removal of one or more stops 25 and the positions of the wheel sets in the slots 24 and 24A so that the panel and wheel sets as a single assembly can be removed from the surrounding frame. MULTI-PANEL EMBODIMENT A multi panel or stacker embodiment of the present invention will now be described with reference to Figures 13 to 30. The aspects of the invention described with reference to the single panel embodiment, of a tracked undercut is present also in the multi-panel embodiment, and is repeated in the sill detail for additional sliding panels located outward from the first or inner sliding panel. The design intent of this improvement to stacker doors or panels is to provide a means of reducing detail in the sill section of a sliding or stacker panel to facilitate easy cleaning and less areas for trapped dirt and construction debris to collect. The aim is also to provide long term durability for the running gear so that it is not as affected as a traditional bottom rolling sliding door during construction and in service. Provided is a nosing 65 with a second undercut 66 beneath it at the outermost point of the sill frame member 17. Again the undercut 66 is inwards and upwards of the outer most edge of the sloped surface 8 of the sill frame member 17A and upwards of the lower most edge of the sloped surface 8. The second undercut 66 is again defined by a downwardly facing surface 81, with an skirting 82 outwardly thereof. This second undercut 66 allows outer bottom runners 67, of which there is one at each bottom corner of the outer sliding panel 54, to hook around and under the nosing 65 into the second undercut 66. This retains the outer sliding panel 54 from being pulled away from the opening 45. The inner sliding panel 53 is retained in the identical way as the single panel described above. The sliding panels, 53 (inner) & 54 (outer), slide on the outside of the door frame 47, this is opposite to a conventional sliding door. A similar higher weather tightness performance rating, to that of the single sliding panel as described above, is achievable with this convention. The head frame, 15 A, incorporates two or more integrated tracks 68. There being integrated tracks 68 for each sliding panel outward from the opening. The jamb frames 16 A, incorporate a channel, 42, for the leading panel stile 21 of the leading door panel to close into. In the preferred embodiment the sill top surface 8 is angled at 2 degrees, though any angle that is suitable for the application can be used and is within the scope of the invention. There is a threshold member 57 fitted to the sill frame member 17A which fills in the void when viewed in plan (e.g. Figures 20 and 21) that is left between the outer sliding panel 54, and the sill upstand area 69. The inner sliding panel 53 travel is restricted by the mullion, 20, and the trailing panel stile, 22, and their interlocking arrangement 43, so therefore that void where the inner panel might travel if not restrained by the interlocking arrangement 43 needs to be filled so as to create a positive barrier between inside and outside to ensure good weather tightness; the threshold member 57 performs this function. A "mohair" or similar type seal 11 known in the art, provides a weather tight seal between the sill frame member 17A and the opening facing (i.e. inwards facing) lower surfaces of the sliding panels 53 and 54. In the single sliding panel 7 embodiment the seal lies along the length of the sill frame member 17. The seal 11 in one form occupies a seal channel 71, which is a known way to retain such a seal. In similar fashion the seal 11 lies along the sill frame member 17A in the multi-panel embodiment. However as each subsequent and successive panel after the first stacks outwards the seal 11 must also be stacked out. Thus there is a need for a member to continue the retention means for the seal 11. The interconnection membe4 61 provides for the continuation of the seal channel 71. The threshold member 57 has the interconnection member or end fitting 61, which provides a retention means for the mohair type seal 11. The seal 11 can be is inserted into the threshold member 57 along it's length. The end fitting 61 has a mating opening to that seal channel 71 of the threshold section. The seal 11 then enters the end fitting 61 at this opening to continue around and butt perpendicular into the mohair seal 11 of the sill frame member 17 A. This threshold member 57 in conjunction with the end fitting 61 and mohair seal 11 effectively form a continuous "Z" shaped seal, when viewed in plan (e.g. in Figure 21) that neatly follows the innermost surfaces of the panels 53 and 54 and critically at the point where both panels overlap 72 and engage 73 together, to form a near perfect air and water seal, which is essential for preventing ingress of air or water. The engaging 73 is identical to the engagement 43 of the single sliding panel 7 embodiment, with the exception that instead of the single sliding panel 7 engagement 43 with the mullion 20, it is now the leading stile of the inner sliding panel 53, engaging 73 with the trailing stile of the outer sliding panel 54. Commonly in other doors, the threshold member 57 would finish without an end cap or returned mohair seal. The inner panel would then finish up to the raw cut end of the threshold section without forming a neat air or weatherproof seal, possibly encouraging water and air ingress. There are further extrusions in the set that allow other configurations of door to be made including overlights 40 and further fixed sidelights 18, as evidenced in Figure 2. Possible configurations include 3 sliding panel (1 fixed panel and 2 moving the same way), 4 panel (two fixed panels either side of two moving central sliding panels going the same way), 6 panel (two fixed either side of two sets of two sliding panels bi parting in the middle), and combinations of overlights (opening sashes or fixed glass above the moving panels and fixed panels), or further sidelights (as many segments of fixed glass or opening sashes to the side of sliding panels as is necessary). The sliding panels 53 and 54 each have at their top have load bearing elements. In the preferred embodiments these again are wheeled. In particular there is an adjustable wheel set 12 and a non adjustable wheel set 13. The wheel sets are fixed at each top corner of the sliding panel. The adjustable wheel set 12 has an adjustment mechanism which allows the panel to which it is attached to be moved up and down approximately 4.0mm. This feature is required to move a panel out of square to compensate for the door frame 47 being poorly installed. If the door frame 47 is installed square then no such adjustment will be necessary. The non adjustable wheel set 13, is fixed solidly to the top of the stile, and has no up or down adjustment. The adjustable wheel set 12 is typically attached to the top of the leading panel stile 21 of the inner sliding panel 53. The non adjustable wheel set 13 is typically fixed to the trailing panel stile 22 of the inner sliding panel 53. Though as stated there may be any combination at either end. The outer sliding panel 54 has the non adjustable wheel carriage 13 typically attached to the leading panel stile 21. The trailing panel stile 22 of the outer sliding panel 54 has the adjustable wheel set 12. At least one adjustable wheel carriage should be fitted per sliding panel to allow for door fitting tolerances, although all wheels of the non adjustable variety 13 would still be possible. The adjustable wheel set 12 can be fitted to both top corners of the panels, 53 and 53, if required, effectively doubling the available tilt of the panels to get the panel's vertical axis parallel to the jamb 16A or mullion 20 of a poorly installed door, where the door may be out of vertical alignment. The wheel set 12 and 13 may have more than two tyres to increase load capacity. The inner bottom runners 14 for the inner sliding panel 53 are used to retain the inner panel to the sill frame member 17A while the door is being operated. There is one guide at each bottom corner of the inner panel. Without these guides, the bottom edge of the door panel could swing outwards once it is disengaged from it's home interlocked position with the mullion 20. The panel could not swing inwards as it is bearing on a mohair sealing strip 11 within the sill frame member 17A. The bottom runners 14 run in the undercut, 9, of the door sill frame member 17 A. There is a friction reducing material 64 in the undercut 9, which is an extruded nylon sleeve for the bottom runner foot 31 to run in for smooth operation and long term durability. Equally the friction reducing material could be present on the bottom runner foot 31. The outer bottom runners 67 of the outer sliding panel 54 are used to retain the outer sliding panel 54 to the sill frame member 17A while the door is being operated. There is one outer bottom runner 67 attached at each bottom corner of the outer sliding panel 54. Each of the outer bottom runners has a bearing means for engagement with the sill frame member 16A. Preferably this engagement is a wheel 76. There is a friction reducing material at the interface of the wheel 76 and the sill 16A. The wheel preferably is made from a hard wearing and / or friction reducing material such as nylon. Without these guides 67 the bottom edge of the door panel could swing outwards once it is disengaged from its home interlocked position with the channel 42 in jamb 16A and the leading edge of the inner sliding panel 53. The outer sliding panel 54 could not swing inwards as it is bearing on a mohair sealing strip 11 within the applied threshold member 57 for part of its travel. After clearing the threshold member 57 it relies on the wheels 76 bearing and / or spinning front and back of the outer undercut 66 of the door sill's nosing 65. The guides in operation are not loaded; they merely keep the panel in line with the sill frame member 17A during panel operation. The guides may come under a pull out negative wind loading when the panels are stationary in their home position. PANEL FITTING SEQUENCE. The head frame member 15A has two or more integrated tracks 68, an inner panel track 59 and an outer panel track 60, which the tyres of the wheel sets 12 and 13 can run in. The throats 23 of the tracks 68 are of course narrower than the gauge of the wheels, so a series of slots, 77, 78, 79 are machined into the head frame member 15A which allows the wheel set 12 and 13 for each panel to be inserted into the integrated tracks 68. The wheel set may or may not be attached to the door panel while being inserted. These slots are strategically positioned within the integrated tracks 68 so that during the panel's normal travel, the wheel carriages will not fall into the slots and allow the panels to disengage from the track. The removable door stops 25 are fitted after the wheel sets have been inserted into the integrated tracks 68 to restrict panel movement in the open direction, to prevent such wheel set disengagement. The inner track slot 77 is positioned near to the mullion 20 such that the inner sliding panel 53 leading panel stile 21 wheel set 12 can travel in the opening direction only as far as the door stop 25 allows it to. The fully open position of the inner sliding panel 53 will stop short of the slot 77. The trailing panel stile 22 and it's wheel carriage 13 can travel in the opening direction only as far as the interlocking feature 43 between the mullion 20 and the trailing panel stile 22 allows it to. The fully closed position of the inner sliding panel 53 will stop short of the slot 77. The second inner track slot 78 is nearer to the jamb 16A and is the insertion point for the wheel set 12 of the inner sliding panel 53. The door stop 25 is fixed over this slot after the wheel and panel has been engaged in the track to restrict movement and stop the wheel set 13 from dropping out the slot whilst being moved in the opening direction. Slots 77 and 78 are placed a distance apart that more or less matches the distance apart of the wheel sets, 25 and 26, of the inner panel, 12, for a simple one step insertion procedure. The outer track slot 79 is positioned nearer to the jamb 17A. This slot, 79, is for engaging the adjustable wheel 12 of the outer sliding panel 54 only. It is not possible to have a further slot in the outer panel track 60 nearer the mullion 20 as the outer sliding panel 54 has a travel requirement more or less double that of the inner sliding panel 53. If an additional slot existed near the mullion 20 the adjustable wheels 12 would fall in to this slot during the opening sequence. The method required to insert the outer sliding panel 54 to the outer panel track 60 involves fitting the adjustable wheel 12 to the leading panel stile 21 prior to attempting panel insertion. The non adjustable wheel 13 has to be inserted through the outer track slot 79 whilst loose from the outer sliding panel 54. The adjustable wheel 12 which is pre attached to the leading panel stile 21 is then inserted through the outer panel slot 79 and the trailing panel stile 22, is sleeved over the non adjustable wheel 13 and the two are fixed together. The second door stop 25 is then fitted to restrict the opening of the outer sliding panel 54 to prevent the adjustable wheel 12 from dropping out of the outer track slot 79. At this stage, both panels are running but swing free at the bottom edges. The bottom runners 14 of the inner sliding panel 53 have been pre fitted to the bottom ends of the stiles 21 and 22 by their lower screw engagement feature 26B which allows a degree of tilt about the radial arc of the screw feature 26B. This tilt allows the guide 14 to tilt over and pass under the undercut feature, 9, and then tilt back up again to engage with the friction reducing material 64 before the top screw feature 26 A is utilized therefore locking the guide 14 in the desired upright position. The outer bottom runner 67 of the outer sliding panel 54 are inserted up the stiles, 21 and 22 while the panel 54, swings free away from the sill frame member 17A. As the outer sliding panel 54 is swung back toward the sill frame member 17A, the outer bottom runners 67 are lowered and the wheel 76 can slip under the nosing 65 of the sill 17A. Once the outer sliding panel 54 is in the vertical position, the outer bottom runners 67 can be raised and fixed into position by their screw features 80. PANEL REMOVAL SEQUENCE. Panel 53, 54 removal is required for future maintenance to running gear or glass replacement for some versions of the sliding panel. The outer bottom runners 67 of the outer sliding panel 54 are unscrewed and are let to drop and contact the sill. This allows the nylon wheel 76 to clear the nosing 65 and the bottom edge of the outer panel can be swung outward, which further allows the outer bottom runners 67 to drop out of the stiles 21 and 22. The bottom runners 14 of the inner panel 53 have their upper screw feature 26A removed which allows a degree of tilt to the guide 14 about the radial arc of the lower screw feature 26B. The bottom runner 14 can swing down until it contacts the sill frame member 17 A. This allows the bottom runner 14 to clear the undercut feature 9 and allows the inner panel 53 to swing out at it's bottom edge releasing it from its constraints. The non adjustable wheel 13 of the outer panel 54 is then unscrewed from the stile 22 which frees that top corner of the panel. The outer track doorstop 25 is removed which allows the outer panel 54 to slide open far enough for the adjustable wheel 12 in the leading stile 21 to drop out the outer track slot 79 freeing the panel 54 from the door frame 47. The doorstop 25 is removed from the inner panel track 59, which allows the inner sliding panel 53 to open far enough for the adjustable 12 and non adjustable 13 wheels to drop out the inner track slots, 77 and 78 respectively freeing the inner sliding panel 53 from the door frame 47. The terms inner and outer with reference to the invention are relative terms. Though there are two panels discussed here they are exemplary only, and there may be 2 or more panels present, the relationship between any two panels being that one is inner (inner panel) to the outer (outer panel).