Right so I need more details before I can say much. What time of night was it, and was there any sound at all? That's the first thing I always ask because a lot of hovering objects people report over water turn out to be drones but drones are almost always audible within a certain range, especially over still water where sound carries differently.
I'm up in Derbyshire and we had something similar reported over one of the Ladybower arms back in 2019, never got a satisfying explanation for that one either.
The reservoir location is interesting to me. There's a pattern with UAP sightings near large bodies of water that doesn't get talked about enough on here. Was it low over the surface or higher up? Any colour to the light, or just white? Did it move at all or was it genuinely stationary the whole time you watched it?
Don't just sit on this, write down everything you remember right now before the details start blurring. Trust me on that one.