The only evidence I've seen of someone getting done, after extensive searching, were two cases:
One was a guy on a Segway riding on the footpath. The other was someone that had a sit-on scooter that had no pedals. We hear the odd rumour that people are being prosecuted, but those reporting have never put any evidence up. I think some of it from time to time is people scare-mongering. The best solution is to get a restricted legal bike and de-restrict it, because nobody can see, and it's not easy to test. Only if you were involved in a serious accident is there a small chance that someone would think of testing it.