A workaround, that I think it might work, would be to install a test forum, restore the backup there and then you can try to do a merge of you rmain forum with the test forum. I have not tried it this myself though, and it would be adviceable to make a copy of the main forum before you attempt the merge.
It was not meant to be ill advised. I thought that that might have been worth a shot under the circumstances. And as long as a copy of the main forum 's database is made before the merge, then things can be rolled back easy.
Thanks all, I guessed it might be difficult, if not impossible, but is always worth asking. The problem we have with multiple copies of the forum is space, so may not be an option. I'm waiting to find out when the last backup was taken.