You have my sympathies. The guy who moved in below me a year after I bought my flat was an utter nightmare and it nearly drove me insane. That silly, 250bpm dance music blasting away 24/7 - mates coming round at all hours of the night and day, and having to scream at each other to be heard above the music. There wasn't a room in my flat that wasn't affected by the sheer volume so there was no escape. Police did nothing. Env Health did nothing. Then he appeared to be dealing - the music would go off, you'd hear him on the phone and them slamming his door and the main flat door on his way out for 5 mins and he'd be back in again. That went on all night too after a while. In, out, in, out. When the wee one was a baby, he would be threatening to come up and kill her if she dared to cry when HE was trying to sleep. The guy below him gave up and sold his flat to escape the nightmare.
And yet - now he's quiet, talks to me, complains about disturbances outside, is the first to come and pay for any communal repairs etc etc. I don't know what changed him, but something did and he's no trouble at all.
Mind you, I just coughed up £100 for a communal entryphone repair three weeks ago and only 2 of the other 7 flats have paid me. Swines. Last time I do anything for anyone else in the stair. It's £12.50 per flat - not exactly a fortune each. Grrrr.
Tonight I'm going to doorstep them again to try and catch them in and get my money back.