Not that i attended this party myself, but Spiza, organisers DO HAVE A DUTY OF CARE. Yes i agree that you dont make people take drugs etc etc, but do you not think that some of the partygo-ers there not only may have been counting on those hours but also may have been left there in the dark from not having lifts etc? If i attended this party, i would have been a little bit upset at having travelled a long way for a party i would have thought would go for a lot longer.
If you say a party is going to go until 1am, then it should go until then, if you arent sure or your permit extends only until a certain time you should let people know about it, that it "MAY Only go until 8.30pm - but we will try and extend it as long as possible" or something to that extent.