I wonder if the "jet" was a happy accident captured during a double exposure hoax.
I mean, compared to a hovering diamond being intercepted by an RAF fighter, I find a double exposure to be a more likely option. But to me, a double exposure is one of the less likely explanations in this case. As several others have already pointed out, the scene doesn't really look like a double exposure, and if there really were six original negatives, it would have been quite tricky to produce.
But this doesn't mean it couldn't have been created in the darkroom. A more straightforward explanation would be someone using some old-school cut-and-paste, reshooting the photos using a repro stand. The result would then be six sequential frames showing the same diamond, with the jet moving between shots. With the repro camera a bit out of focus, I think disruptions in the grain pattern could be effectively hidden. Early on when investigating the case, I had my suspicions that the whole thing was created using some old press photos of a Scottish landscape showing a low-flying jet. Perhaps even photos originally found in the DR archives

But obviously, there's no proof of such a thing.
Still, I think cut-and-paste is a possibility, but personally, I find the models-on-strings hypothesis more straightforward.
I know of no evidence to support either possibility, but there are so many different ways these pictures could have been produced without the need for any mysterious and inexplicable diamond shaped thing in the sky, there is not much justification for assuming they are genuine at this point.
Indeed! And that's the whole point. With our current understanding of the case, it could be a hoax created using models on strings, a hoax created in the darkroom, or a reflection in a calm lake. It's even possible that it's not a pure hoax at all, but simply two drunken teenagers who mistook a mountaintop covered in clouds and mist for a flying object.
All these possibilities have one thing in common: they don't require anything extraordinary to have occurred. As long as we don't know the identity of the photographer or the circumstances surrounding the creation of the photographs, we have no reason whatsoever to assume that they depict a huge, hovering, diamond-shaped craft.
And this is the fundamental problem with the field of "ufology" in general. "It could be an alien spacecraft—or one of Bob Lazar's reverse-engineered sports models!" Sure, it could. In the same way, a wave on Loch Ness could be the monster lurking beneath the surface. Or it could just be a wave.