Oh, forgot to say that the vet says that both mother and baby are healthy, but he can't tell the sex of the baby yet. We're going back in two weeks time to find that out. In the meantime we've christened him/her Niblet, as it's a tiny Nibbler.
We're keeping them both apart from Leela for a few nights, but putting them together in the evenings when they can be watched. I'm sure she won't, but there's a chance Leela could attack the baby.
Also, I've just seen the cutest thing while washing up: mother and baby popcorning their way around the cage together. They look as happy as Larry.