The display seems to have subsided now (20:00 UT) but I just wanted to jot down what I saw whilst it is fresh in my memory. I am in a semi rural location near the Norfolk Broads. Probably about 18:35 I went outside to look at the sky and there was an arc of milky light running almost due east to due west which I thought at first was cloud. I went to grab my camera and the first image I took was at about 18:42. This showed it was definitely an aurora. The arc ran through Perseus in the east, then probably up to 70 degrees above the northern horizon then down to the west. It was quite well defined. Above it in the east near the horizon was a red glow that showed up well in an image. What was really interesting was that as the arc strengthened in intensity curtains of light travelled along the arc from east to west. I was surprised at how quickly they were travelling. I would say that they covered about 15 degrees of arc in a few seconds.
This is only the second big display I have ever seen so my technical knowledge of how to describe it is poor. The arc dissipated about 18:45 and then returned again to the same strength. I took photos until about 19:10. A bit after that I could see some reddish glow due north (my dinner was ready so I had to go in!). Pictures to follow.
