After noticing some swelling last week, we'd taken him along to the vet's, where the summer locum gave him some antibiotics. These hadn't done anything, so he'd had a different batch at the end of last week. While Jeoff was doing OK, the swelling hadn't gone down, and we could feel something hard under the skin. We took him back to the vet's this afternoon, and the vet (now back from holiday) rushed him off for an X-ray.
The resulting image wasn't good news - it turns out that what was causing the swelling was a tumour in one of the bones of his foot. It looks like it hasn't spread beyond there, but these are aggressive cancers (the vet reckoned that it had only been six weeks or so to get to where things are today), and there's really only one thing we can do to prevent it spreading. So tomorrow Jeoffroy will have to have his leg amputated.
We're completely shocked, but this will be the best thing for him - even if he does find it a little harder to get around in the future. I don't suspect he'll be climbing over the rooftops for a long while.
I'll take him to the vet's for the operation early tomorrow morning, and we'll see where it goes from here.