In reply to morganator:
I have been up a few times and either stayed at the main hut or dossed below the east face (numerous options ) which is on the traverse round to the bivi hut. If you are just after the west ridge, you will have to do the traverse in the morning if dossing or staying at the main hut, or in the evening if staying at the bivi.
We chose option one, as it gave more options for climbs pre and post west ridge day. I recall topping up with water before the bivi hut but it was in a brief lull between bad weather , so not much use for your original question.