well I came home from the vet without her.
Vet really didn't know what was wrong with her but she had a lot of muscle loss and was quite thin, I suspected she hasn't been eating much this week.
When I get back her two old buddies were making a wailing type of noise and looking everywhere for her. It's strange but this morning before I took her one was laying either side of her, it was like they knew.
The vet didn't think she was suffering but said what you had all been saying and that they can deteriorate really fast so I think it was the kindest thing to do as she was more lethargic as the week went on.
Even though our chickens are not really pets and I didn't think I was that attached to them it still made my eyes well up! The vet will take her to the other practice later and give her some gas rather than an injection.