I have never been able to decide what the "right" course of action is in cases like this.
On the one hand it is polite to approach the person yourself and raise the issue informally.
On the other hand, they agreed to terms for their licencing, and went beyond them. There are services such as noise abatement, EH, and others to deal with this, so why not use them.

Its an awkward one, as if the matter turns ugly, and gets into a bit of a battle, the person in the wrong knows exactly who "grassed"

Tough one, but hope the matter gets dealt with amicably, and so there are no losers.