Outlook 2007 – rogue appointment

I have an appointment in Outlook which I cannot turn off.

When I try to dismiss it I get a dialogue box which says:-
Cannot turn off reminder. You may be reminded again. Cannot locate recurrence information for this appointment.

This comes up every time I start Outlook and I have to close the reminder dialogue box to use Outlook.

Most annoying.

Anyone know how to fix this?

Many thanks.