I did consider pulling over, but there didn't seem to be many suitable stopping places. I'd much rather she had overtaken me and disappeared into the distance! I've been driving for over fifty years, all over Britain, without (touch wood) ever having had any sort of accident. I also keep up my speed to the correct limit so as not to delay people behind me. It's a good tip to take the number and report it, as she could be doing this to everyone and cause an accident. We get a lot of elderly drivers who only ever do forty mph, but I'd never do that to them, it's IMO terribly dangerous. Time for a nice cup of tea and some crumpets. Plus, my old friend I'd been visiting had made me a ginger cake to take away, so a slice of that will calm me down no end!
Thank you all for your advice!