Reservoirs come up more than people realise with these sightings, there's something about large bodies of standing water that seems to attract this kind of activity. I've had two reports from Essex alone in the past few years near waterways.
What did the movement look like - was it completely stationary or doing that slow drift thing where it looks still but you can tell it's adjusting position slightly? That detail matters a lot. Also worth noting the time and how long it stayed before it disappeared.
Don't second guess yourself on this one. Get it written down properly while it's fresh, every detail you can remember. What did it look like up close, any lights, sound or lack of sound?