I would call 101 and let the coppers make the call on whether to act or not. Fr instance, if that were my town, where there's usually four or five police cars parked up in the centre for general duites, a nip down to the park to have a quiet word with the owner might be possible Early on a saturday morning.
I think calling the police is extreme and is a waste of their time. It's not right and the owner is a moron BUT the emergency services are really stretched and this isn't an emergency in any way, shape or form. Seriously, try my suggestion, it will work.
It doesn't have to be an emergency to involve the police, and "calling the police" is not necessarily dialling 999. If the local beat bobby or pcso or whatever they are now gets involved now it may well prevent a real emergency if things escalate, either dogwise or owner/runnerwise.
i know you will have a different opinion on what is and isn't important to a police officer, but we don't not go to the GP when we have a cough just because someone else has just has just had a heart attack in another part of the health service.