Outdoor Chair This invention relates to an outdoor chair and in particular, but not exclusively, to chairs for outdoor activities such as angling, hunting etc. It is known to provide chairs for outdoor activities which comprise a seat portion, a back portion and generally splayed legs in the manner of a picnic chair. It is also known to provide such chairs with storage pockets or pouches attached to the seat and/or back portions. However this type of storage is not particularly convenient or flexible as the storage pockets or pouches are down to the side of the user or behind his back. There is therefore a need for a chair which facilitates attachment of accessories for convenient use by the occupant of the chair. Accordingly, in one aspect, this invention provides an outdoor chair, said chair comprising a frame assembly defining a seat portion, a back portion and a plurality of leg members attached to, or forming part of, said frame assembly, wherein at least some of said leg members extend substantially vertically in use and are adjustably mounted with respect to said frame assembly such that an upper portion thereof is exposed. In this manner, the exposed, substantially vertically extending, upper portions of the leg members are available for the attachment of accessories such as trays etc for holding angling accessories or bait etc or items such as rod rests or umbrella fixing brackets. Such accessories can conveniently be attached to the exposed leg members by suitable clamp arrangements for vertically adjustable movement. The term substantially vertical refers to the orientation of the leg members when the chair is placed on a notional flat horizontal surface, with the seat portion generally level, and may in use be several degrees either side of vertical in particular applications. Typically there will be four leg members which extend substantially vertically in use and these may then be arranged as a rearward pair adjacent the back of the seat portion and a forward pair adjacent the front of the seat portion. Preferably the exposed upper portions of at least the forward pair of leg members project substantially vertically in use to either side of the seat portion. This means that the upper portions, and any accessories clamped thereto, are at a convenient height for a seated occupant to the left and right of where they are sitting. Each leg member may advantageously be attached to the frame assembly by a bracket with the leg member being slidably movable and lockable with respect to the bracket. In this way the chair may be adjusted so that the seat portion is horizontal for use. The chair is preferably movable between an erect operational condition and a generally compact stowed configuration. Thus the leg members may be mounted to be foldable between an operational configuration on which they support the seat portion off the ground in use, and a stowed position in which they lie generally flat with respect to said seat portion. Likewise the back portion may be movable between a flat, stowed configuration and one or more erect operational configurations. Locking means may be provided for locking the legs in the operational condition. Likewise, one or more bracing elements may extend between the forward and rearward legs in addition to or instead of the locking means. At least a substantial portion of the frame assembly may conveniently be made of metal tubing such as steel or aluminium. Whilst the invention has been described above it extends to any inventive combination of the features set out above or in the following description. The invention may be performed in various ways, and an embodiment thereof will now be described, by way of example only, reference being made to the accompanying drawings, in which:- Figure 1 is a schematic side view of an embodiment of an angler's chair in accordance with this invention; Figure 2 is a front view of a generally similar example but with certain parts removed for clarity; Figures 3(a) to 3(c) are side views showing different bracing or locking configurations, and Figures 4 to 8 are respective views of a further embodiment from the side, rear, front, and above when open, and from the side, view when closed. Referring to Figures 1 to 3, the chair 10 comprises a pair of side rails 12 interconnected by two transverse members (not shown) to form a subframe of generally rectangular form. At its four corners the subframe is attached to four generally vertical leg members 14 by hingeable leg brackets 16. The leg brackets 16 allow the leg members to pivot from the operational, vertical, configuration shown in Figure 1 to a condition in which they lie flat and generally parallel to the side rails. The leg bracket preferably includes a releasable locking mechanism to lock them in the vertical configuration, for safety. The leg brackets 16 are slidably and lockably mounted on the leg members 14 so that the effective height of the leg members can be adjusted, and leaving an upper portion exposed. Leg members 14 are each provided at their lower end with a load spreading pad 18. A backrest portion 19 of inverted U-shape, is secured at its free ends to respective reclining brackets 20. The reclining brackets 20 allow the backrest portion to be moved from a compact stowed configuration in which it lies flat with respect to the side rails 12, to the erect position shown in Figure 1. The reclining brackets 20 releasably lock the backrest portion in this configuration and may allow the backrest to be locked at different angles of rake. In use, a suitable padding/upholstery will be applied to the backrest portion and likewise adding an upholstery will be slung between the slide rails to provide a seat portion. As seen in Figures 1 and 2, the leg members 14 are of sufficient length to support the seat off the ground, but also leave a portion thereof exposed above the leg brackets 16. These exposed portions are designed to act as mounting posts for one or more accessories such as the trays 24 which are mounted on the exposed portions by suitable clamps 22. Referring now to Figures 3(a) to 3(c), there are shown various modifications to the arrangement shown in Figure 1. Figure 3(a) shows an arrangement in which a brace frame 30 is pivotally connected at one end to the rear legs 14 by a collar 32 and has a loop or eye 34 at the other end which hooks onto hook brackets 36 secured onto the front legs. In Figure 3(b), a toggle catch arrangement 38 is provided to lock the brackets 16 against hinging with the toggle part being located on the bracket 16 and cooperating with a hasp on the rail 12. In Figure 3(c), lock pins 40 are provided which the user pushes through aligned holes in the bracket 16 and the side rails 12. Figures 4 to 8 show a further embodiment of chair 110 which comprises a pair of side rails 112 interconnected by forward and rearward transverse members 113 to form a rectangular subframe. In the four comer regions, the side members 112 are connected to respective hingeable leg brackets 116. Each leg bracket includes a hand wheel operated clamp 117 device through which passes a respective leg member 114 which can be adjusted and locked at the required height, thus allowing for levelling of the chair on an uneven terrain. At the foot of each leg member is a load-spreading pad 118 or 'mud foot'. A backrest portion 119 of inverted U-shape, is secured at its free ends to respective reclining brackets 120. The reclining brackets 120 allow the backrest portion to be moved from the erect position shown in Figure 4, to a compact stowed configuration in which it lies flat with respect to the side rails 111 shown in Figure 8. The reclining brackets 120 releasably lock the backrest portion 119 in the erect configuration, and may allow the backrest to be locked at different angles of rake. — In use, a suitable padding/upholstery will be applied to the backrest portion and likewise adding an upholstery will be slung between the slide rails to provide a seat portion. As seen in Figures 4 to 6, the leg members 114 are of sufficient length to support the seat off the ground, but also leave a portion thereof exposed above the clamp 117. These exposed portions are designed to act as mounting posts for one or more accessories such as side trays, feeder arms, pole supports and so on.