Thanks Deggie, but I've done that now (having seen that suggested for error 1602 and 1604) and it's failed on both attempts.
1. I get the Recovery process started.
2. It went right though the 5 min procedure of 'Extracting software' the first time. (The second time this seems to be skipped.)
3. Then the text changes to 'Preparing iPad for restore'. But nothing is happening. iPad screen is blank.
4. Then after about 5-10 mins, I get an error message 1604. I looked like that 1602 message I got earlier except that the name Terry's iPad is now iPad. (I've crudely shown it below, after hacking it in my image editor.) That seems significant to me. Somehow because my iPad's name has changed? Or because it's trying to restore from a very old backup?
Anyway, reading up on error 1604 I see no mention of anything like that. But I wonder if I can hack some text file that will sort this out?
I'm rapidly reaching a state of despair!
Terry, East Grinstead, UK