Mate this kind of thing messes with your head doesn't it. I had something similar a few years back over the fields near my place in Cornwall, just this silent shape sitting above the treeline not moving, no sound at all. The silence is what got me more than anything.
What shape was it roughly? And did it move off eventually or did you lose sight of it? Those details matter I reckon. A lot of people assume its a drone these days but drones have that buzzing noise and they dont just hover completely motionless like that for long stretches.
Would love to hear more about this one. Cornwall seems to have a fair bit of activity and I know other parts of the UK do too, feels like theres been more reports lately or maybe people are just less embarrassed to talk about it now. Either way glad you posted it up.