Thanks for your reply Fleecewife. I think we are going to try putting up some sort of netting/mesh, obviously nothing that any bird could get stuck in. Might also try dangling CD''s around which might put the owl off.
On the other hand, yes we are lucky to have the owls. They're very feisty little birds and have been around here for a long time. I think the main problem last year was that it was so dry, they were unable to feed on things like earthworms as the soil was rock hard.
Anyway I hope this year, we get the swallows and owls breeding as usual, but the young owls being fed on worms rather than baby swallows!