Thanks for your advice. I called a vet, and took her up this lunchtime - the vet wasn't really sure what it was likely to be, but she estimated she had at least 900g of fluid in her abdomen, and she was very skinny underneath it all.
she said most likely causes were either a heart condition, or some sort of tumour, and neither of which were really treatable in chickens, so we did the kindest thing and had her PTS.
She did give me the offer of taking her home and doing it, but i thought about it, and decided i'd rather face that another day! Still got a lot to learn...
Anyway, thanks for your post.
ToGGoT