@summerdown, yes we did, thanks for asking.
We had a chat with DS's nursery about it the day before, and they told us they thought Luke will actually be going to one of the local village schools. So when we went in, the first thing the head (who was lovely, btw) asked was if we knew who had bitten DS, and we explained that DS was adamant it was Luke but that now we weren't sure as we'd heard he wouldn't be going to that school. We told her is full name and she checked the class lists and he isn't one of the children!
So it's a bit of a mystery - clearly DS was bitten but we don't know who by. We hypothesised together that perhaps he's so used to beiong bitten by Luke that he automatically says it was him. She explained that the children from all three classes (Hawthorn, Pear and Oak) had been allowed to mix at playtime so we couldn't even be sure it was another child in Hawthorn. She said they would be happy to move DS into either of the other classes if we wanted them to, anyway. (In my email I'd suggested they might be better separated but hadn't actually asked to have him moved, and she mentioned that she was aware that I wasn't DEMANDING anything). But we chatted about it with her for a bit, and bearing in mind that several people have told us the teacher from Hawthorn is the nicest reception teacher, and we couldn't even be sure it was a Hawthorn child who bit him, AND it definitely wasn't LUKE so no previous history between DS and the biter, we decided to leave him where he is for now. Especially since he does have at least one friend in Hawthorn and has seen his classroom and met his teacher now.
She said they hadn't seen him be bitten nor had he told anyone (probably because he didn't know who to go to), and they seemed genuinely horrified and embarrassed about it. She said if they had seen it, they would have called us about it, and they also send home an accident form when children are injured in any way. She said they completely understood why we were unhappy about it and would keep a very close eye on the class for further incidents, and that if there are any further incidents, we can look again at moving DS.
Overall, we were very happy and reassured. We realised that they do have procedures in place if it happens again - she also said to contact her again if there are any similar "serious" incidents (which made me feel like they were taking it seriously) or even if DS feels left out or bullied in any way. And she seemed really responsive and friendly and approachable. So fingers crossed it was just a one-off prompted by heightened emotions at Transition Day.
Thanks for all the advice people offered on this thread, it was very helpful.