Hi, well I rang the vet and unfortunately the amount of money they wanted just to get antibotics was astronomical and we just arent capable of spending that much right now. I did however go to my local pet store and they had a broad spectrum antibiotic on the shelf. So I got that as I have been really upset that we were not able to get professional help. I then did some research on line and discovered that some vets have actually prescribed this antibiotic before. I also wondered if giving aspirin would help take away the pain for her and found out from a research site that giving chickens 25mg of aspirin per pound of body weight per day severely reduces the death rate from peritonitis. You can also add vitamin E and this will help even more. So, what I have been doing is giving aspirin first up in the morning then half the antibiotics at about 10 at lunch more aspirin at about 5 more antibiotics. She is not too happy at the site of me I can tell you that. But she is still alive, she seems to be happy is eating drinking and I her tummy has reduced in size. So maybe it is a slow process of getting better. I am also still giving her baths. A lot of effort, but she is a lovely chook (especially considering the run is up behind the house up a steep hill - I am fit now!)and I think I am helping her get better.
If she has peritonitis (she has been like this for over a week now) would she have died by now if the infection was not being stopped?

I pulled this off another thread which may be of help to you.
I have used Baytril orally for my hens with great success. Dose rate for that is .5ml for a normal sized hen, 1x daily.

Do a search for antibiotics in the search field (Top right of screen by FAQ. You should be able to find a lot of information on various kinds of antibiotics
All the best for you and your hen.