I've said it before and I'll say it again, the ultimate control lies with the electorate. But they don't care until they have a negative experience personally. The parish precepts form a tiny part of the council tax bill, often less than a quid a week per person, so why should they care, until something goes wrong? There are a million uninsured drivers on our roads, but that only matters to me when it's my car they crash into. Otherwise, I turn a blind eye to their law-breaking and get on with my life.
Giving principal councils an enforcement role would make a difference, but the workload would be unimaginable (nine frivolous or misguided complaints for every valid one) and the cost unaffordable. In England alone, we have 307 district, borough, and unitary councils that would each need a couple of extra staff. Let's say, for argument's sake £100k including oncosts, etc. I'm not sure there's a spare £30million in the system right now, or that this would be the best use of it if there were.
One final thought. Where would they find these 614 people with the necessary knowledge and experience? Former chairmen of parish councils perhaps?!