Sounds like you have a sick chicken. You will need to check out your vets attitude to chickens, many won't treat them as they are so fragile, but instead put them to sleep, as treating with anitibiotics will mean the eggs are worthless and when they recover they are usually carriers. Sadly it is usually far better to cull and stop the other chickens from getting ill.

To be brutally honest £35 can buy an awful lot of chickens! And I doubt it will cover a pre-existing condition.

Hope the hen is better soon