It's still an unusual place for planes to be.....
Thankyou for all the replies.
One of the points in my first posting was that for 2 days in succession, jet aircraft had been in places in the sky above where I live, when I had seen no jet aircraft in those places before. The only things that fly down the A355 are some of those big heli-jobs out of Denham. (?Queens flight?) and I know they ask ATC for the OK. I've never seen jet planes in that place before. Maybe the odd single engine prop. out of Denham, but not jets........
As an aside....
ZJ691 used to be painted a very boring light grey, presumably to reduce how often it was noticed. The one I saw had a much flashier appearance, so if it was ZJ691, someone from MoD has splashed out on a flash paint job.