Well the update saga for my iPad 2 has come to a very satisfactory conclusion at last.
I initially updated to iOS 8 from iOS 7.1.2, and that's where the problems started. I did the update over wifi, which I now believe was a bad idea. Everything was as though the iPad had developed Alzheimer's, it was slow to respond and things were missing. I managed to revert to iOS 7.1.2, but Sync failed in iTunes and I couldn't turn on iCloud backup ( it kept throwing me back to the home screen).
So today after a lot of hair pulling I bit the bullet and used iTunes to update to iOS 8.0.2. I was very wary at first, but it all went smoothly.
I really can't believe it, it's as fast as it ever was, there's no appreciable lag (except maybe for iBooks ), I've tuned iCloud backup on again, and if anything, for most things the iPad appears to be faster (if that's possible).
Sorry for the long post, but I'm over the moon that I don't have to spend out for a new iPad yet!
Steve