I forgot to mention that over Fram we were buzzed, ten o'clock high, by an all-silver Yak (3M, I think) with a red star on the fuselage, which turned and came past again. You can see why I thought it was a Spit at first...until it banked and I saw its wing shape, which looked oddly like a Hurricane's. We then thought the fuselage, with that wing, made it a Mustang, which would have been much more exciting than a Yak, of course, but it was still a real highlight sharing airspace with this character.
Just found this on Google and it looks like the very one. It was seen at Duxford last year in the Flying Legends show, so there is every chance it is the one we saw.