The fact the neighbours have complained about the volume, thus being a nuisance to them surely provides the evidence that the volume is excessive?
Not really, you could argue that if you can hear anything then the volume is excessive. That doesn't mean it's a reasonable claim.
I can hear my neighbour cough occasionally when I'm in bed at 1am - should I go round and complain about the excessive noise?
Also he has stated that he had "12 people" in the flat, now unless these people had taken a vow of silence, walked on sponges or were just playing musical statues then i'd imagine there would have been noise.
12 people in a flat, drinking(?), with music on that is loud enough to annoy the neighbours, it can be assumed that those people in the flat would have to raise their voices to communicate over the music. This doesn't seem like the recipe for a tranquil scene to me.
So by that argument, it's not the music that's an issue - it's the fact that the OP had friends over. At 6pm. On a Saturday.
Not expecting this to happen occasionally is just as unreasonable as the OP expecting to be able to play his music out as loud as he wants.