Thanks to everyone who came up with suggestions. My survey is booked for just over a week's time.
For me, I think this is the crux of it. I've had a look at the boat - it seems well built and in the time I spent on it I saw nothing wrong. But I don't have a huge amount of experience of such things, and although I've been boating for quite a few years, I've never been responsible for maintaining a boat (other than running repairs). So I probably wouldn't spot problems that might be obvious to others.
I'm paying someone with a great deal of experience, to take an independant look. He'll also not have the 'rose tinted glasses' that I will have had when I found a boat that felt right for me. If he finds something wrong then I can bargain with the vendor, or I can walk away.
If he finds nothing wrong, then I can be happy that my gut instincts and own observations were probably ok. If something later turns out to be a problem then I may have some come-back, but this is probably tricky. Which is why I wanted a recommendation for a surveyor, rather than take pot-luck that I don't use a cowboy (if such things exist). It's in a surveyor's interest to do a good job and to get it right - his reputation counts for a lot.
Unfortunately the terms and conditions do tend to limit the possible comeback against the surveyor if he misses something, it excludes things that can't be seen, and the inner-workings of the engine - just a visual inspection of it, rather than any sort of dis-assembly. But if he is negligent, and can be proved to be so, then he would certainly be liable.
So for me, it's a lot of money, but as a proportion of the purchase price it's peanuts, and having someone 'in the know' look over it for me is worthwhile.
Thanks again for everyone's input.
Paul (hopefully afloat by Christmas )