Sorry for going AWOL, I went down with a nasty cold during the afternoon and have only just lemsip maxed myself back into life.
I can't honestly say it wasn't a prop plane of some sort. You kind of accumulate a bundle of memories over time that don't necessarily connect up and I have some thing in my head that says Viscount was a possible type I flew on. But as a rule the fact of planes being jets was built into the tour offer, and I have relatively consistent memories of these early flights. And because my feeling was that we flew Gatwick with KT in a comment on the first trip out (in 1971) and subsequently from Manchester, and the records from Manchester are superb, with only 727-46 G-BAEF flying on this route on 21st May 1973, I think I just had a bit of a premature QED. There's a lot about this that still makes limited sense, and one part is which direction the plane was flying in the "tip" photo and the direction of the approach. I'm on the fence about whether it's a tail fin or a wingtip, and I'm really not concerned about the apparent shape of the window as there may have been some distortion due to the lens or the direction my dad was shooting.
1-11s were a very common type on the route, with various tricks to eke out the range,, and I'd flown on one the preceding year (absolutely no question about this, the whole wing is visible, see below). We chose the packages based on the hotel we liked, which was the Messonghi beach hotel, and I suspect that if that had meant a drive to Gatwick, we'd have done a drive. So I'm checking what flew out of Gatwick too now, where the records aren't nearly as good. I don't believe we ever flew from Luton.
I don't seem to have a working "like" button, but very grateful for everyone who's posted, and I would be hammering on it. When I have new information I'll post it, but please continue with the speculative ideas.
The Alps from what I believe was a Laker 1-11 in May 1972