In my country there would be one of 2 issues here (or both):

A Disturbing Noise is objective – a scientifically measurable noise level and generally compared to the existing ambient noise level.
A Noise Nuisance is a subjective measure and is defined as any noise that “disturbs or impairs or may disturb or impair the convenience or peace of any person.

So looks like it would be Noise Nuisance and have no idea what this entails in the UK, since you are saying that the disturbing noise is below the threshold (10db) which deals with the disturbing noise levels.

You are more than likely dealing with someone who normally gets their way by saying she will call the council.

Good luck