Thanks to all who were quick with advice and offers of help.
I did a half-hour's finger-walking with the Yellow Pages and found the exact HDD 10 miles away in a small/medium computer company.
Kind chap had formatted and checked it before I got there and showed it to me working. Asked for £5 for the biscuit fund. Got home, and it checked out OK on my system so I swapped the contoller/interface card to my broken HDD and it worked a like a dream. So total cost was £1 over budget at £11 (plus petrol).
Got the data. Of course it may just have been blowing the dust off the platter that did the trick.
This was a good learning exercise; the most important lesson I've re-learned that we all know but don't always remember is:
when someone brings you an ailing computer that is just about working, get the data off while you can before you do anything else!
Thanks again.