My mother once rescued a fully adult sea-gull that had broken its wing.
She kept it in the largest birdcage she had - one my father had made and it was large, fed it on cat's fish and water, much to the disgust of the cat, I may add. Said cat had his own ideas about birds, which didn't include keeping them in cages.
Peter, as we called him, recovered and returned to the wild, but came to visit now and again. We knew him because although his wing bone knitted properly, he had a bare patch where no feathers grew.
The last time he came, he had another seagull with him.
We did sometimes wonder if he was male or female.
Hope your gull does well too.
If the chimney isn't in use, do get a roofer to put something over the top.