Thanks for the thoughts and advice everyone, I really appreciate it. We talked about this a lot last night. DPs initial reaction when he saw my arms was "Oh he has to go back". But my neighbour (v. experienced dog owner) came over and we all talked then the rescue lady phoned and talked it through with me. I do want to give him another chance because as I say it 100% wasn't malice, it was play - BUT that being said, I do have to think of my own safety (and others, if he were to try it with them). I really do bruise easily so I'm completely used to having bruises but that said, this was more powerful that normal 'mouthing'.
Basically, if it's a not a behaviour that can be nipped in the bud FAST with training, then he will need to go to someone more experienced. Because even though he won't seriously harm me, we'll never bond if he is able to hurt me so we wouldn't be helping him. He was so baffled and worried afterwards, bless him. I think one of the reasons I was so upset after it happened was because I really love him and want to keep him.
The rescue lady told me exactly what to do next time he tries it: say NO! loudly and firmly, slip lead straight on, lead to conservatory, shut door between us for five minutes for him to calm down, then bring him out and do some sits/paw for treats. And every single time he starts, he gets put back in timeout so he learns its not fun to do. Sound so simple and logical looking back on it!
So I'm taking it hour by hour today. Loose clothing does seem to be a trigger so I've whacked the heating up and I'm wearing one of my dance unitards today. And the trainer is phoning today to arrange a time to come, and if he can't come soon enough I'll just ring someone else and get them to come.
I did say to DP when he got him the only thing that would ever make me return him would be if it were in some way unsafe (for any of us). At least if we did have to return him, which I don't think we will, he will have have at least had the small benefit of having been loved and cared for and warm and safe while he was here, as opposed to in a kennel on his own.
He had his first walkie on the beach this morning with me, my neighbour and my neighbours two dogs and he loved it. He was so amazed by the waves!