Update on the puppy: they brought her back the same day. She arrived back at 7pm after a 5 hour drive, had slept all the way back, bounced back into my house and jumped on her brother (who is leaving for his new home this weekend) and started playing! I had expected a rather subdued puppy but she was obviously well looked after in the 2 days she spent with them. I thanked them for bringing her all the way back and they were very apologetic. I suspect the reality of a puppy in the house made them think again even though when I 'vetted' them they had obviously done their research. Apparently this is a new house (they had had a dog until he died 6 months ago, so were experienced dog owners).
The puppy seems none the worse for 2 long journeys in 3 days and it is as if she was never away. Playing with her brother, mother and our 2 other dogs, eating well and bright eyed.
Luckily I had an email from someone that afternoon asking if I knew any puppies available. She had come with a friend to pick up a puppy from me last week and was looking for one. To cut a long story short, she is an ideal puppy owner, came to look at the wee girl the next evening and is taking her home at the end of the week (I want to keep this puppy a few more days to make absolutely sure she is fine - another change of environment in a couple of days would not be good, I think). The new owner has a holiday booked in June for week but I'm more than happy to have the pup back for that week as I'm sure she will give the girl a lovely home. She (the pup) will see two of her brothers regularly as they will all be in the same puppy class!
So all's well that ends well. Phew! Been a topsy-turvy few days!