I was travelling home from work in the early evening. When I stepped onto the platform at Peckham Rye I saw that all the other travellers were facing the same direction, staring into the sky. It looked like a scene from Close Encounters. Following their gaze I was mesmerised too, by a glowing, shimmering oval sitting next to the setting sun. One of my fellow travellers suggested it was a fire on the ground reflected in the clouds. That didn't seem very likely, but Londoners' cynicism triumphed, and I didn't hear anyone suggest that it was a flying saucer, even though it did actually look like a flying saucer.
I thought it was possibly a variant of the Northern Lights, or another weather phenomenon, so when I got home I looked up the phone number of the Met Office, so I could ask them about it. I picked up the phone to ring them, and then realised I'd sound like an eccentric reporting a UFO, so I didn't.
Thanks to Wikipedia, I now know it was a Sun Dog. They are quite common in Alberta where I live now.