Just hit the log off button, and received the following message
An Error Has Occurred! Unable to verify referring url. Please go back and try again.
I have 'keep me logged in permanently ticked' but can't think of anything else that might be relevant. I have also visited two other SMF boards in the same IE7 session. The http://usertools.plus.net/mods/index.php one, and PUG. Thought I'd make a note in case anybody else has a similar experience.