In reply to pec:
Thanks to pec, and to all responders - I will now follwo the plan of bivying in a snow-hole out on the Vallee Blanche, though will bring the tent with if needed. As we will not be leaving for any ascents on the morning after, rather just using the altitude for acclimatisation, I'm not worried about being moved on or having the tent confiscated (We'll be in it). I will also use the time to check out the previously mentioned hut, though I do not envisage using it - will make for an interesting detour as part of the trip.
Thank you all - once again, ye provide a priceless service to those less in the know
Hope to be able to return the favour in the future to some on here.