That kind of sighting really does stick with you, doesn't it. Something about seeing it above the treeline specifically - that height and the way it just sits there - makes it hard to rationalise away.
I had something similar about three years back, just outside Bristol. Oval-ish shape, completely silent, no navigation lights, just hovering for maybe 40 seconds then gone. No dramatic exit, it just wasn't there anymore. That part messed with me more than anything.
Can you describe the shape or any light coming off it? And roughly what time was this? I've noticed a lot of these treeline sightings happen in that window between 11pm and 2am which is interesting. Would love to hear more detail before jumping to any conclusions.