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Many thanks for all the helpful feedback!
I think I will start by contacting the landlord and electricity provider, followed by a letter to the neighbour forewarning him of the consequences of a repeat. It would seem that it is a civil matter, but I'm wondering if it counts as Criminal Damage. Proving it happened to the police will not be a problem as there were three of us here. Our other neighbour will probably be glad to vouch that it has happened to him as well.
The fuse box is shared between the flats, so there's not much we can do (physically) to stop him from having access.
As for the sound levels, the levels in our living room were low enough for normal conversation to be heard over the music, however, the problem lies with the low frequencies that travel upstairs. Other neighbours have said they cannot hear anything, but this chap lives directly above us. Again, we're happy to cooperate to find a level that doesn't bother anyone, but he's gone about opening dialogue in just the worst possible way.
Thanks again guys :)