Thanks for all your advice! Before reading this thread again I'd been out to buy her some kitten food as I had a pregnant mum on that last year & it seemed to help her (and the RSPCA recommended it).
I'm on a very low budget so switching to a high quality cat food isn't really an option. I'd love to though!
We're going to cook up some chicken for the week & she's going to get some of that.
I also gave her some cat milk earlier which she wolfed down (just a small amount) and am feeding her small and often.