In reply to Damo:
Altberg only make up to B1 and its a soft B1 at that. The Mallastang (which I use for summer), no where near as stiff as a Scarpa SL.
Cedric, I would take a look at B3's as well, The La Sportiva Nepal in particular is pretty well known for its large sizes, Needle Sports in Keswick stocks the largest sizes if you live near there. I used a Scarpa Manta for a while, found it didn't really fit that well and got the Nepal, which is more comfy to walk in even though its stiffer. Whatever you buy, make sure it fits your foot.
Another thing to bare in mind too is that the largest sizes make the boots more bendy than smaller sizes as there is more length in them to lever, even a Nepal would flex slightly in say a size 50.
Good luck finding something.
Craig.