That'll almost certainly be a juvenile Long-Eared Owl ... a pig to find 'cos not only do they (usually) wait until after dark to start calling, but it'll be at the top of a tree and it turns it's head as it calls so the sound seems to come from different directions as you get close. I was fortunate to find a nest locally about 20 years ago that was in the top of an oak that was growing near the bottom of a 50ft bank ... from the top of the bank we could see straight into the nest ... just like on the telly 8)
The adults knew we were there, they came over to have a look, but didn't seem worried if we stayed still. One of the adults (can't remember which) has an alarm call like a dog barking ... quite un-nerving from halfway up a tree in the dark !!