Apparently my accountant has known about it since November 2006, but she was "hoping I wouldn't have to pay it". It looks like it was triggered by a change from contracting to running my own business (with three friends) in August 2006. She's advised me to pay it within 30 days (i.e. take a hit in my next pay packet), as it's already overdue. If I don't I'll end up paying 1.5 x the figure (or something like that). I think either she's screwed up or my old umbrella company did. I'm going to look into it properly on Monday...