Well thanks for the replies!
Stu666 - nope, not a KC-135R. I did consider drones, but pretty sure it wasn't a prop drone - Taranis did cross my mind though, but on the wild assumption that it would sound a bit like a Hawk being fitted with an Adour I ruled that out, although as it's LO it may also be noticeably quieter than a Hawk.
DaveF68 - no, not a C-130. The MV-22 suggestion is interesting, I've never seen one flying, so not familiar with the sound. I didn't hear it as it approached, just as it was more or less directly overhead & then heading away - would an MV-22 sound like a turbofan aircraft from behind when in full forward flight (no blade noise) or would it sound more like a Turboprop?
To elaborate, whatever it was flew straight & level right up the middle of The Great Glen, West to East / Fort William headed in the Inverness direction. Little chance of there being any confusion with commercial traffic & this flies almost exclusively South to North or vice versa heading / returning for the US. Light commercial or GA at night is more or less a non event in the area, the occasional SAA King Air at medium altitude or Biz-Jet high altitude, more or less only Mil, SAA EC135 or SAR after dark.
If it was military, Lossiemouth would seem to be the most likely destination (maybe), but no unusual movements noted on Fightercontrol