Thank you all. Although still a very poorly bunny Bunita is improving. She is slowly eating chopped veg and herbs. She even showed interest in hay today but couldn't eat any.
She is still very quiet but her ears are now up more often than not rather than flat and she is going over to the water bowl rather than waiting for me to put it next to her.
We still have no idea what's wrong but it seems clear the antibiotics are what is helping her. As none of the three vets that have seen her can find the problem, I am not sure what I can say to the first vet. I think we were just lucky yesterday that we saw Juliet, it was her first day back after maternity leave, and as an exotics vet she has a good gut instinct.