He was a camera enthusiast (Pentax SLR), and used a lot of lenses including wide angle and telephoto, and not unlikely he was using wide angle out of the window, it would make sense for landscape shots I think. Sadly now deceased, so I can't ask him!
If this was a Manchester flight, it was a 727 somehow, but really losing confidence in that possibility now. I'm asking someone who has records from Gatwick to see what went up on the date in question (21st May 1973)