IMPROVED MASONRY BLOCK TECHNICAL FIELD This invention relates to building elements such as construction blocks and has particular relevance to masonry blocks and buildings having walls constructed therewith. BACKGROUND ART There have been proposed many different forms of masonry blocks for use in building constructions and these are generally manufactured with a peripheral wall with apertures passing therethrough. The blocks, when built into a wall, are generally laid staggered, that is with the blocks in alternate rows being displaced half a block with from those in the adjacent row and are normally provided with mortar joints. Such blocks have the advantage of lightness in weight and are less costly than bricks. Additionally, in staggered block constructions a level foundation is required for supporting the blocks to ensure correct alignment between the blocks. Furthermore, the blocks are required to be manufactured to stringent tolerances otherwise staggering of the blocks becomes difficult. Building blocks are generally manufactured by machinery that tamps a moist mixture of concrete into dies whilst the dies and/or a table beneath the dies are vibrated. As can be readily understood such techniques result in wear and tear to the dies and product which was made from a new die would not necessarily be dimensionally the same as product manufactured subsequently. Difficulties arise when the blocks are laid in multiple courses because of this problem. Australian patent No. AU-A-24868/95 describes a method for manufacturing walls from masonry blocks having end apertures opening to the exterior of the block, the methodology including the steps of arranging blocks in a substantially end-to-end butting relationship so that the apertures are adjacent and oppose each other, interconnecting blocks by a plug or dowel adapted to extend through the openings and having portions receivable in the respective apertures so as to hold the blocks in a required position relative to each other and setting further blocks in alignment one above the other. Blocks produced in accordance with Australian Patent No. 735489 provide useful construction elements However, the blocks produced are often susceptible to chipping around the edges, may be difficult to manufacture with consistent filling of the moulds and are often difficult to grout. Whilst the blocks described above have proved to be satisfactory we perceive that there are a number of improvements which could be effected the block design to aid in the manufacturing processes and in the use of the blocks in constructing a wall. DISCLOSURE OF THE INVENTION According to a first aspect of the present invention there is provided a masonry block comprising outer walls, at least one abutment wall and at least one vertical cavity, at least one first channel in the upper surfaces of the abutment walls and/or in the lower surfaces of the abutment walls, wherein the abutment walls include vertical edge surfaces for engagement with abutting blocks, a vertical land disposed between the vertical edge surfaces wherein said land has a keyway for receiving a key to lock adjacent blocks in position and a pair of vertical channels disposed between the vertical land and the edge surfaces, the arrangement and construction being such that when blocks are laid one upon another the channels and the said at least one cavity of the blocks can be aligned to provide an internal network of horizontal and vertical grouting passages which can be filled with settable material to seal joints between the blocks and to provide load bearing support between respective layers of blocks. The masonry blocks may be of a variety of configurations. In one form the masonry blocks include the abutment walls at opposed ends of the block such that the block may be used to form straight wall, hi another form the abutment walls may be on adjacent sides of the block such the block may be used to form a corner. Another form of the block is with a single abutment wall such that the block may be used to form an end to the wall. The masonry blocks of the present invention may include one or more vertical webs such that a number of cavities are formed within each block. Preferably the vertical webs are of a corresponding dimension to that of the abutment walls such that the continuity of the grouting channels is maintained. The masonry blocks are preferably formed from concrete and manufactured in moulds. We have found that by having the walls, webs and other sections to be of 25 mm in cross section or more the moulding is made more reliable in that it is less likely that voids are moulded into the blocks rendering them of less structural integrity. It is preferred that the walls, webs and the like are of about 30mm in thickness. The outer walls are preferably formed to be supported by and to support other layers of block. It is preferred that the outer walls have stepped upper and lower surfaces such that the blocks are locatable on and can locate adjacent layers of blocks. In so far as the blocks have a first channel, the outer walls are preferably braced where they extend below the abutment walls and any webs. Suitable bracing may be in the form of a thickened section that may be tapered from the abutment walls and any webs. The said at least one first channel be substantially in the top of the abutment walls and any webs and may be substantially U-shaped. Alternatively said at least one first channel be substantially in the bottom of the abutment walls and any webs and may be substantially an inverted U-shape. In the preferred form the first channel be formed in both the top and bottom of the abutment walls in adjacent layers of blocks where, in the top of the abutment walls and any webs a U-shaped channel is formed and in the bottom of the abutment walls and any webs an inverted U-shape may be formed. In an inverted channel on the bottom of the abutment walls and any webs it is preferred that the channel is divided by a central ridge. The abutment walls have vertical surfaces for engaging adjacent blocks. The vertical surfaces are preferably the same thickness as the outer walls and may be formed on the ends of the outer walls. The abutment walls include a vertical land disposed between the vertical surfaces. It is preferred that the land is centrally positioned between the vertical surfaces. In a preferred configuration the land is recessed relative to the vertical surfaces such that a grouting passage extends between the respective lands of adjacent blocks in abutment. The lands include keyways for receiving interlocking keys. The keys can each comprise an elongate medial flange and webs positioned on opposite edges of the flange each of said webs including a plurality of ribs which extend from the sides of the flange and which, commencing from the outermost ribs are of diminishing depth. The surfaces of the flanges of the keys can be irregular. The irregularity of the flange surfaces of each key can be provided at least in part, by a network of extending knobs. The irregularity of the surfaces of the flange of each key can be provided at least in part, by a network of recesses. According to a further aspect of the present invention there is provided a wall structure comprising multiple courses of masonry blocks aforesaid in an end to end abutting relationship including keys fixed in the keyways of the blocks. The wall structure can include including vertical and/or horizontal steel reinforcing members positioned in aligned cavities and channels in the blocks. Horizontally disposed passages and vertically disposed cavities within the structure can be grouted. The invention will now be described with reference to the accompanying drawings, which have been provided for illustrative purposes and are not intended to limit the scope of the disclosure herein. BRIEF DESCRIPTION OF THE DRAWINGS Aspects or the present invention will now be described with reference to the accompanying drawings in which; Figure 1 is a perspective view of a masonry block according to one embodiment of the present invention; Figure 2 is a top view of the masonry block shown in Figure 1; Figure 3 is an end view of the masonry block shown in Figure 1 ; Figure 4 is a bottom view of the masonry block shown in Figure 1; Figure 5 is a side view of the masonry block shown in Figure 1. The masonry block 1 shown in Figures 1 to 5 is formed with a pair of opposed sidewalls 2. The opposed side walls 2 are spaced apart by abutment walls 3 and a web 4. The top of the abutment walls 3 and the web 4 are arcuate so as to form a horizontal channel that extends along the length of the block and can form a grout passage. The bottom of the abutment walls 3 and the web 4 are similarly arcuate so as to form the top of the grout passage. Lobes 6 extend downwards from the respective abutment walls 3 and web 4. Figure 2 shows the configuration of the abutment walls 3 in which the vertical lands 7 extend from the end walls 2. The land 8 is vertically disposed between the respective vertical surfaces 7 and includes a keyway 9 in which a key (not shown) can be inserted to retain adjacent bricks in a locked engagement prior to grouting. The land 8 is recessed relative to the respective vertical surfaces 7 such that a grout passage is formed between the respective lands on adjacent blocks. Vertical channels 10 are formed between the land 8 and the respective vertical surfaces 7. Figure 4 shows a brace 11 that buttresses the side walls 2 and minimises chipping or damage when in use. A similar brace 12 is located on the top of the block 1 and buttresses the top edge of the side walls 1. The top of the side walls 1 have an upper step 13 and a lower step 14 to engage with a foot 15 on the base of a side wall 2 of a block on an upper course. Persons skilled in the art will appreciate that the invention described above may be subject to improvements and modifications that will be apparent without departing from the spirit and scope of the invention described herein.