Don't worry about it, it isn't a compulsary thing. They do it now and again, sometimes they may see an irragularity that spark off an investigation and at other times it is just a random check. If you can't remember, just tell them, or, don't reply to the letter.
1.6 tdci, remapped to 120 bhp, Panther Black Titanium with rear view camera, nav pac and front sensors. Always willing to stop & help in any situation I can. Karma is a beautiful thing.