Dear 'Somebody Help Please' !

My mum and dad moved into a retirement bungalow 2 years ago. From almost day one the daughter of the neighbour who owns next door has constantly played loud music throughout the night. She has physically threatened my parents and even 'promised' to kill them if they continue to complain. Of course, we have contacted the Environmental guys and the Police who try (?) but at the end of the day I/we are now at our whit's end with the whole situation with 2 elderly parents and their neighbour who is laughing at the law and continuing to stick two fingers up to everyone. I fear this is now making my parents ill, my mother is already suffering with cancer .. the woman next door doesn't care, the Police cannot do anything, Environmental Health are toothless; they record and record and record... but ultimately the woman next door knows the system and doesn't care.
To add to the situation the neighbour responsible for the noise has a long list of previous, including an incident of stabbing which made the local newspaper 7 years ago.
My parents have worked heir whole life and are absolute genuine caring citizens & the neighbour just does not care ... she is killing them, the authorities cannot (or will not) do anything concrete.
This is a sad sad situation where it seems once again the perpetrator has all the rights --- never worked, doesn't care & hates the world. My parents, ill, absolutely would help anyone in their hour of need and never had an incident with the law in 77 years are tossed aside.
No idea where to go next.... any ideas?
PLEASE HELP!