What to do?

Harold lives in our street. He isn't thought of as a stray, just a homeless cat. He is fed and loved by a handful of people. He has never wanted to live in anyones house, he seems to like his independence and come and go as he pleases. The last few days I have been very worried about him as he is limping and not wanting to put weight on his left legs. In this photo you can see he is leaning on his right side.
I tried to call the cats protection league today to see if they can help with vet costs, or have any advice, but I just left a message on an answermachine.
Tonight, for the first time ever, Harold actually came inside our house, just into the hall. But Magnus was not happy, and growled at him. He stayed for about 20min, then  left again.