I haven't.
I did see summat on the telly a while back where they were talking to several people who had fallen foul of the system though.
One was a lady who had lost control of her car into a ditch and the assessor noticed they were not the std wheels for that model (although the std size) and that she hadn't notified them of this 'modification'. Unfortunately the wheels had been like that since she bought it (second hand) from the dealer and were an an 'upgrade' option.
Another was a guy who had his car stolen and when talking to the assessor / loss adjuster answered the " and have you had any issues before this " with "well, they did try to steal the radio once and must have been disturbed so I just got the window replaced and another time when they did actually get the radio ..". Because he hadn't notified them of these events (or was stupid enough to tell them at this time) they invalidated his insurance for being 'under insured' (they would have put his premiums up had he notified them of the other losses etc).
When I fitted a tow bar to the motorbike I got confirmation IN WRITING that the insurance Co were both aware and happy with said modification. I also mention tow bars when re-insuring the cars, if they ask or if there is a box on the eform.
Cheers, T i m