Thanks chaps. Although I completely agree with NT's view that a self- build would be more robust, I suspect that it will be more straightforward to get something from a supplier and 'help' it with a little careful modification.
I looked up the supplier that David mentioned and they do provide a good reference point for price, which is much keener than a B&Q special or the like, with a great deal more choice. Jackson's, suggested by Peter, are well within the area and I'll be giving them a call when I have a moment to myself. Unfortunately, their online price list is dated 2007 and they appear to be offering supply and erection which, while attractive, makes things a bit more complicated from my point of view (ie having to be at home and having the base prepared ready for them). Of course, I'd rather go with a local small firm if they can do what I need.
I'm intrigued at the idea of using fence posts as bearers (David and Chris), as I was thinking in terms of having to make a slab base: I was sort of hoping to use up a pile of old bricks as hardcore, but I suppose I could do that just as easily under fence post bearers, without using as many slabs. It does make sense, as it would lift the whole thing good and clear of the base to prevent rot to some extent. Just wondering if I can get a stack of 2.4m 75 x 75s in the car (oh how I miss the last one, that could take a bigger load!)
I can see Harry's proposition of using soffit board. It wouldn't rot and you can get it in other colours from some sources, though I think it wouldn't be as rigid as T+G, so the frame would have to be much sturdier, with more uprights to nail it to. Of course, it wouldn't rot, so that would be a bonus....
Cheers