In reply to Tim Sparrow:
I first met those owls great, great, great grandparents in 1971 leading Oakover. I put my hand in a crack and heard a hissing noise that sounded like a snake. After almost falling off in fright I exited upwards rapidly convinced I was going to be chased by an angry adder. Two years later I seconded it and coming to the same crack heard the same hiss, but this time behind me. Turning round, expecting to see Kaa from the Jungle Book I was mightily relieved to see a baby owl sat in the oak tree hissing at me. It's good to hear that they're still nesting successfully in the same area.