Update
I thought I'd share an update since you've been so kind in helping Ruby through her illness. Since I put her back outside, she's ever so slowly improved. For quite a while she was still thin and her comb was shrivelled and grey, although she seemed to still have an appetite and seemed happy. In the last 3 weeks or so, I've noticed she seems to have put all her weight back on again and it seemed her comb was growing a little and getting more colour.
About a week ago she was scratching around in the nest, arranging it to her liking etc. Finally, about 4 days ago, she started laying again! She's so far been laying every day.
Thanks again for all the advice and support.
LD