Can anybody tell me if the wheels have to be a certain size for the SVA inspection? I eventually intend to fit 15 or 16 inch wheels with low profile tyres. Also how will fitting wider wheels, possibly with a different offset to ford standard, effect the steering geometry?

The effect of fitting wheels of different width and offsets, may add up good or bad depending. If its bad then wheels might foul the suspension or be way outside the support of the wheel bearings.
The book 'how to build and modify sportscar and kit car suspension and brakes' by Des Hammill gives a good grounding in this subject
John
PS dont know about the sva stuff, though I expect that as long as the wheels weren't silly, unsafe or protruding from the bodywork you would be ok
John

as long as the tyres are legal, and got correct speed ratings your ok, but i would tend to stear away from using too low a profile tyre, most of your suspension will come from the tyre wall, ok for tracks but on the road they will shake your teeth out, i'm sure anyone who as gone that route will back me up on that one