Thanks again for your support, but I'm sorry to say the poor thing died this afternoon.
We took him to another clinic this lunchtime, and they were far more professional than the last. They said that it seems the first doctor gave the puppy too much worm medicine, and subsequently poisoned it. By the time this was discovered it was too late. They gave it a drip for a while, but an hour or so after arriving home, his heart stopped. :(
As you can imagine, we are both very very sad. We are filled with anger at the first doctor (it seems he isn't a 'qualified' vet), but there's nothing we can do about it.