Question Author
I've checked the sound files on the RSPB site and feel sure they were Tawneys. I can't get to the visitors centre as the entrance is right round the other side of the site (about 8 miles away),and I have to go to work, but I have researched further and found the answer........
"The most vocal time for Tawny Owls is the autumn, when the young owls from the previous spring are all distributing away from their parental territories. All of these fit young Owls move from place to place in their quest to establish a territory of their own, in which they can hunt and eventually breed. They will inevitably, however, come up against many existing territories along the way, which is when the resident birds will call to defend their area against the new batch of intruders."