Here are my own thoughts:
Mirror image/reflection in water.
There are a few debunkers who are speculating that the symmetry of the "UFO" (for want of a better word) in the image could be caused by a reflection in water. What they've forgotten about is where the photo was claimed to have been taken and ask themselves if there is a) a body of water in the vicinity which could provide a reflection and b) can that body of water be in the camera's lens/view.
Loch Tummel is in the direction of the camera lens, but it's far below at some 630m below Farragon Hill and miles away from it, thus a reflection in water is not going to produce any symmetry.
The aircraft, not the UFO.
I'm pretty sure the aircraft in the image is real. It does look like a Harrier, but it also looks like a Hunter. As far as RAF operations are concerned, this area was not one where anyone would expect a Harrier of whatever mark to train/fly and the MoD said there were no Harrier aircraft flying in the region at the time. Well they would say that wouldn't they!! However, that would make sense. If someone asks and the answer is no, it's not a conspiracy, but what about Hunters or even USMC Harriers? I had a brief google and 237 OCU (Operation Conversion Unit) Squadron were flying Hunters from RAF Lossiemouth at the time. Occam's Razor would suggest that's the likely case and some bugger has a logbook showing such!
Is there a possibility the aircraft is a USMC AV8B? I haven't spent the time to look into that, but I know there are far more knowledgable people than me who will know.
So how did such an image come about?
Having seen the video above, which matches the surrounding terrain and having looked into it as per my post above, I'm convinced that the photo shows the top of Farragon Hill with a Harrier/Hunter in the foreground heading along the valley and going west to east along one of the valleys.
I've done a good bit of hill walking and mountain biking in Scotland, albeit 25 years ago, but not in the region the photo was taken. There were plenty of times I saw low flying aircraft, including a pair of Tornados, way below Ben Etive coming into the valley via Loch Etive, as I was climbing up. You felt you could reach out and touch them. Obviously the aircraft photographed was much further away, but, as we all know, it's not unusual to see military aircraft at low level in Scotland.
I think the noise of that aircraft made them look in the general direction and see something that looked weird. The two chefs claimed that the object (UFO) was stationary for 10 minutes and that the aircraft circled around. The top of a mountain doesn't move. It's silent and not man-made. The only thing the top of a mountain doesn't do is shoot straight up having been stationary for 10 minutes.
I think there's much more to this story and it isn't aliens or secret US aircraft. I think the pair heard the aircraft, turned and watched, saw something they couldn't immediately explain, got spooked and took photos, but, because of the weather conditions at the time, the photographic quality of the camera and the way the negatives were produced ended up with what we see today and the idea of the UFO shooting straight up was part of them being spooked.
I've no explanation as to why the mountain top ended up being the only part of the photo, along with the aircraft, which were seen so clearly (along with the fence and overhanging tree) in the resulting photo, but I think that's simply the camera, the media, conditions and the way the negative was processed. We can only see the camera result, not the human eye.
I think this photo is genuine, but the interpretation is wrong as far as wishing this to be an alien craft or a 'black' US project.