@Devilshands Thanks both of you. Fair points and all taken on board.
We're no longer leaving him alone, haven't done since the two-hour incident. (Admittedly, this was a bit of an emergency situation, but still.)
Appreciate the advice to take it back to the beginning. To give a bit of context, we've left him alone for a total of five times - 10 minutes, 20 minutes, 30 minutes, one hour and two hours. No incidents during the shorter periods. I accept this is too long.
The puppy has been in three homes. His original owners sadly experienced a sudden death. He was loved and we were told that they had started leaving him alone for short periods, although I have no way of knowing if that is true. They didn't have a crate for him. We rescued him from a small charity that deals with cats and dogs, and they all go into foster homes. - so no kennels for him. He was in a foster home for two weeks before coming to us.
We have created a safe puppy comfortable space/room for him with nothing dangerous to hand (or so we thought) but we didn't realise he could open doors until the last time we left him alone, so that was a bit daft and naive of us.
We aren't anti-crate, it's just not something we have used before with our past Boxer pup.