Sorry folks, this week has been a real nightmare for the server - it was behaving so well for months, with weeks of uptime. 12 hours seems to be a struggle now! I've taken some drastic action and upgraded everything I can think of to the latest version, as it's seems to be a compatibility problem with a PHP module. I've tested this combo out on my own PC and it worked fine, but this is a high higher volume loading so I'm hoping it performs as well.
We'll see over the next few hours, but fingers crossed this does it. Nothing had changed apart from auto upgrades/patches for months, so a recent security patch seems to have caused it.