Has anyone else had further problems since then?
I've found the cause and the solution isn't ideal. One of the bits of software on the server is incompatible with the operating system (Windows 2008 R2 x64), so I'm going to have to change it. I've got another server ready to move the site to, and I estimate the change over should only take a few minutes. Switching servers is the only way to avoid downtime when changing OS, but luckily the servers are only on a month-to-month rent, so I can time the overlap days by just a day or two :D.
I'll do it early UK time so that there aren't many (if any) members online at that time. I'll be able to sync the database like last time, so there won't be any posts lost.
I'll post back here when this is complete :). At a guess, I think the switch will take place on Thurs morning. I just want to test the server a little more to make sure that there are no other bugs or software incompatibilies.