Logged on to the server this evening to find it sending large amounts of spam. Clearly something on the server has been compromised and, as a result, the sheer amount of emails being sent out has caused the site to become unresponsive at times. I'm on the case and will update shortly.
As an FYI SPAM was directed at general addresses, not addresses logged in the forum database. Probably, once again, a Chinese or Russian spammer.