Thanks everyone.

@Sherwood, I'll try your suggestion first I think. Thank you.

@Snazy - "Hope you get to the bottom of it" haha! I agree with you, I don't want to start accosting dog walkers on the street, that'd be very unfair because I bet most of them are responsible.

@Beige - I'd be very annoyed too if it was on my property. In my case, it's just on the public path outside my house - but that's infuriating enough!

I'm going to try @Sherwood's advice first, I may also write a large polite notice in chalk on the path outside my house.