Last night I was staying on a campsite on the Wentloog Levels near Newport, South Wales (UK). At about 22:00 (dusk) I started to hear an unfamiliar bird call, clearly more than one individual, and looked out. Sadly I had no binoculars. The habitat is farmland and grassy coastal floodplain, with scattered trees and hedgerows (including close to where I saw the birds) and is rather wet.
In the end I saw three individuals at the same time - two perched together on a fence, and one flying. It was pretty dark so I couldn't get a good look, and the two I saw perched were against a dark background. The impression I got was of a darkish bird, lighter underwings visible in flight. The shape was good for an owl, both in flight and perched, though perched it could have been any stocky raptor.
The size was comparable to woodpigeons that were also around (and may have been flushed by the owls) - that's good for Short-eared Owl (known to frequent the area but mainly/only in winter), Long-eared Owl (resident), Tawny Owl, and Barn Owl (the impression I got was darker than adult barn owls; I also heard a classic barn owl screech, when I was about 100m away and the sound came from further than that).
This is a recording on my phone. This doesn't match any of the owl calls I can find for the candidate species. The combination of an unusual tone and what may have been a family group suggests juveniles to me.
Can anyone help me pin down the species? I'll be back there in a couple of weeks and can take my binoculars, but if these were fully fledged juveniles they may not hang around with the parents much longer.