They are going to put him under anaethetic and try to clean the ear out thoroughly today. Fingers crossed it works.
EDIT: 4:30pm: just had a call from the vet: the abcess has ruptured: they have cleaned it out, but he may not make it because it's fairly advanced.
I am not entirely happy that the abcess wasn't picked up the first time I brought him in, when the ear was already discharging pus, and that they put him on Baytril for 5 days instead of dealing with it straight away. 5 days of an ear abcess is not good. I think now I should have pushed harder for more to be done, and taken him back on Mon, not waited till Weds. Easy to say now. :-(
EDIT2: They have sent him home with more Baytril, to come back on Sat. I also have some Gentamicin ear drops, to be administered 3 times a day. He has also had injections of Antisedan, Baytril, and Rimadyl.
His eardrum is burst and he is having difficulty balancing. I asked if we should be giving him something stronger and was told the injections should sort it.
I am not feeling a lot of confidence in this vet. So far, the guys on the Refuge forum reckon he should be on Septrin, or possibly Duphapen (penicillin), and Rabbit Rehome suggest penicillin, an x ray to find the source of the infection, and an operation called the zepp procedure.
I think tomorrow I shall call the local rescues and ask if they can recommend a really good rabbit vet.