I was wondering that meself last night, and came to the conclusion that it was a calibration flight. Here's another one, today monday 22nd Nov, a very low flying Piper Aztec (or similar) doing circuits over SE London, rear left side hatch open and, i assume a cameraman/photographer going about his business. G-A*ZL or similar was the registration using a mark 1 eyeball. Any info, just curious
SHJ