Yes, it all went smoothly. Dexter was a superstar, they took him for a little walk without us, and he went off happily. Jumped all over all three of them, no shits given. He didn't bite anyone - result!
They're clearly very keen to do the best by him. The Dad asked if we would be interested in meeting up with them, and bring Luna, at some point. Of course we said we'd be delighted!
So, yes, it's weird he's not here now, and I'm nervous that it all works out in the short term, puppy biting wise, but I'm really happy he's with them.