In reply to saz_b:
The standard answer, and the one I went with for Ama Dablam, used to be IHI. But, I think something has changed about it now which makes it less attractive, hopefully someone who knows a bit more will be along.
It was the cheapest cover for high altitude I could find, and you didn't have to get it from home country to home country, unlike many other insurers (WARNING, many policies are invalidated if you don't get them for the entire trip from home and back, and they'll use this as a reason to decline a claim, its really fairly obvious from your travel tickets.)