Looneytunes is absolutely spot on here!
The local Environmental Health Officer would initially send out 'standard' letters to both you and your neighbour. Your will probably include diary sheets so that you can monitor the noise.
If there is a specific problem out of office hours then they will have an 'out of hours' contact number that you could use but this may vary from one Authority to another.
After the neighbour has had a warning, if the noise problem is continued and, this is key, witnessed by an officer of the council, then a noise abatement notice can be served on them.
Following this, if the noise still continues, any breech of this notice can result in the equipment that is used to cause the noise nuisance being seized which will more than likely be followed by legal action (a court summons).
If there are other obvious anti social behaviour issues to go with this, the authority may request an anti-social behaviour order from the courts(ASBO) ... or something similar.. a CRASBO (Don't ask!! lol)
But for your part.. all you need to know really is to contact Environmental Health, get diary sheets, complete them and send them back as requested.
One other thing.... Write down on the sheets how the noise is affecting you at the time... such as... I was sat watching the television at a reasonable level and could only hear the base from a song that was playing... (if you know the song, write down what it is). If you had to go out of the house for peace and quiet, write it down.
Don't hold anything back or feel that you are 'bothering' the EHO because many people feel this way and don't bother calling which means that the case cannot be proven and will no doubt be closed de to lack of evidence.
Hope this helps... Good luck!
Cheatseeker!