Thanks very much ShetlandPaul, a bit of straight talking is just what I needed. The chicks legs and feet are normal, not curled under or splayed, just weak and the chicken is probably quite old, she has large white scales on her legs which are thick and knobbly - I am told this is a sign of age. I moved to the house and took on the existing flock so I am guessing their ages. Now I know what I need to do, if I want to keep chickens I reckon I am just going to have to do what needs to be done.
Thanks again, this site really is a gold mine.