It was over 4000 files or something, can't remember. Seemed a bit excessive.
Deleted all of the session files as best I could without a proper ftp client.
Added a gc_probability (or something) suggestion from here.
Have had a discussion with the hosting provider over twitter and they say they haven't changed anything. So there shouldn't even have been an upgrade. I don't believe it though. But the support answer on email was even less helpful than the twitter discussion. I'm thinking about giving them a call next week and if I get any more information out of that I'll post it here.
