Saw this the other week and checked my car, came up as not on the database. It then occurred to me I put a different plate on about 3 weeks after I bought the car (March 2007) but forgot the tell the insurance
called the insurance immediately and they corrected the details (I even told them I'd changed the plates 7 months ago) and it is now on the database.
Makes you wonder though, I have done 11,000 miles since I got the car, mainly motorway M6/M1 and I've not been stopped!!!