Thanks Hoagy,
Perhaps I did miss it, but I don't have any record in my email history, and it is unlikely that I deleted it.
Their email yesterday says that 'You are receiving this e-mail as you have not renewed your ROS certificate since the 24th March 2010.' This tells me that I did renew on 24th March.
The date is suspiciously close to yours, but I can't see their usage to mean that I did not renew on 24th.
Anyway, it's my problem, thanks for the comment.
I mentioned it here to make other forum users aware, in case they get a surprise when they log on.