In reply to williemilne:
Based on my one visit to the Balmenhorn Bivi Hut at that time of year I'd say there was a good chance it won't be full. For some reason everyone else seemd to prefer paying for a noisy overcrowded experience in the nearby manned huts whilst we had the bivi hut to ourselves.
If it takes your fancy it's got blankets, a stove, gas and pans etc, just take your food. Oh and the toilet is something else!