Involve the police, the dogs need to go on record and the owner be approached formally to ensure something is done. Otherwise you may find you get a few quid to replace the hens and it happens again because they think that is all they had to do. That's my personal view anyway. Partly because if I approached the dog owner they would likely be defensive if not aggressive and I wouldn't need that on top of losing the hens in the first place. And partly because not knowing who they are at first is your best protection against bad feeling if they then ask why you didn't go to them instead of making an official complaint. The dog that is 'in custody' as it were, would be taken in and the owners found/informed, then it's nothing to do with you, it's between the owner and the law as it should be (in my opinion).