I've told Direct Line each time I've swapped winter wheels & tyres on and off - they're quite happy to amend my policy without charge or penalty. My only concern is that the OEM fitment on my car is staggered 18s and the winters are 225/45/17 all round, so if I was involved in an accident with them on and they didn't know then they might claim I'd invalidated the policy by modifying my car without their knowledge. Seems paranoid I guess, but we do all go on about what robbing bar stewards insurance companies are....