Hi Baz,
My last garage was a 20x10 timber building. It had a single engineering brick base, and a damp course membrane. I got a slightly larger roof (so the roofs were each an 8 foor sheet, so it had a larger than normal overhang.
That garage rocked! It was naturally warm and dry, no condensation, and as the car was a little Seven, plenty of room to work.
I brought it in '97 and it is still going strong now, I haven't touched it maintenance wise in that time.
Don't think that a timber garage is a compromise, my grandpa built one in the 1930's for his Model A lorries, he moved and remodelled it in the 70s, reused the railway sleeper floor and made it into a 30x15 workshop. It is still standing. As a boy I loved being in there, the smell of timber and cresote, the ticking of an old factory 'clocking-in' clock he had from his factory as his shop clock will be with me always.
So I reckon go for a timber garage. The great thing is, you can take it with you when you move! That and also the walls are a bit thinner than brick or block, so slightly more room inside.
I got mine from Warwick Garages, I liked how the framing on theirs was more chunky than others, and the whole thing is lined in 1/4" ply, and then the weatherboarding on top. They are great quality. Check out their website.
http://www.warwickgarages.co.uk/
If I have another 'overflow' garage I will have timber again. Just check the quality of a few before you buy.