Right so I've been lurking on this forum for ages waiting for someone local-ish to post something like this because I swear I saw something similar over the downs near Avebury about three weeks back. Just sitting there above the trees, no sound, no movement, then gone. The "no sound" bit is what gets me every time - helicopters and drones you hear coming a mile off but this was just silence.
What shape was it roughly? Did it have any colour to it or just dark against the sky? And did you get that weird feeling like it knew you were watching, because I had that and I know how mental that sounds.