That description of the hovering before it moved off is what gets me - the stationary phase is almost always the part people fixate on and I think thats for good reason. When something is just sitting there, your brain has time to rule out the easy explanations one by one.
I had something similar over the Somerset Levels about three years back, low and completely silent, and the thing that stuck with me was how it seemed to almost drift rather than fly. No rotor noise, no nav lights in the right positions, nothing that matched conventional aircraft behaviour.
Few questions if you don't mind - roughly what time was it, and what was the light quality like? Colour, pulsing or steady, did it change at all before it moved? Also which direction did it go when it eventually left?
The tree line detail matters too because it gives you a rough altitude reference which is useful for ruling out drones. Most commercial drones you can hear within about 100 metres, especially at night when ambient noise drops. If it was silent at that range you're into genuinely interesting territory.