Not ripping on Pope (RIP), but he was what, a mid-level bureaucrat in the '90s? How many mid-level bureaucrats have recently passed away? Who's heard of any of them, besides Pope? He was a nobody who rode the UFO scene the 2nd half of his life with books, and appearances. My Facebook page was inundated with memorials for him, and I don't follow any UFO pages. I always found his response to Clarke uncovering one of the photos interesting, if nonchalant. He had hyped it as one of the best UFO and commissioned a somewhat fanciful recreation, but when the real thing showed up and was less than compelling, he kinda shrugged. In the end, he didn't know anymore than we do.

As for the witnesses, I've never understood how anyone can give much credence to their claims. According to Lindsay, he spoke to 1 person by phone, once. He wasn't even face to face and can't really know who he was talking to. There are hints that some other MoD people MAY have spoken to the witnesses, but it's unclear. AFAIK, the local newspaper had no record of interviewing the witness, though they may have. The witness name on the photo has led nowhere. There is no record of the supposed 2nd witness, aside from the story told to Lindsay.

The only possible scenario I can see where the craft was some kind of US technology, would involve a lot of confusion or deliberate misstatements by the witnesses. That is, they saw and photographed some sort of stealth-like project, which still flew like all other known stealth craft by conventional means and either they thought it hovered or concocted a story that it hovered.

As @Mendel pointed out, even if it was some sort of secret craft, it has antecedents that likely date back to the '70s. So called "breakthrough technology" is largely a myth that gets tossed around in UFO circles to excuse the lack of antecedents we would expect for "exotic" aircraft.

In addition, if it was a secret aircraft that had been deployed, hence it showing up in Scotland, it would be known, if not then, certainly by now. The history of secret aircraft confirms this. The U2 was developed in secret, but once deployed, it's hard to hide it. Its missions were secret, but the US listed it as a NASA high altitude research plane. The Johnson administration spilled the beans on the A12/SR71 soon after it went operational. Same with the B2 and F117. People are going to see these things. Not so with the Calvine craft.

It appears Clarke has either gone down the path of media interest with his secret aircraft theory, or he's been convinced by some of his anonymous sources. His on the record sources say it was a hoax.