Single note repeated through the night

Hello.

Would be incredibly grateful if anyone could help to identify this single-note call that we're hearing repeated through the night - every few seconds, for an hour or so at a time.

Just one 'coo'; slightly beepy / electronic-sounding (thought I'd left the fridge door open or something like that to begin with!).

We live in South Wales - next to a river, not far from forest, so we do get some unusual sightings.

Bird in question sounds reasonably large.

Thanks!