I don't want to go back and have it happen again. The owner is that dozy he'll probably let it out the next time i'm there which is my main worry. If i know its locked inside then i'll be happy,but i think it'll attack someone else. It's more a question of when not if.
In all honesty then, if that is what YOU want, then I would suggest...
Visit the owner. Explain exactly that. Explain that you have considered reporting the matter to the police, but all you actually want is that guarantee it won't happen again. If he's then amenable, work something out. Perhaps you could knock the front door before going round the back of they are in - and I assume the dog is always in if they are out?
He may not wish to listen, he may react badly. You then have your own decision as to simply lose him as a customer, or to report him to the police and claim compensation, or both.