In reply to shibz:
+3 for plastics. I bought some Vegas. As people say you can sell them off again later. Mera can be very cold - when we were there in October 2007, somebody got airlifted off with frostbitten feet, through having poor footwear. You might get away with it, but I can't see how it could be worth the gamble, considering the costs of getting you to the point where you need them, only to have to turn back for the sake of a few quid.
We were on the white cold stuff for at least 3 days, up to camp on the Mera La, then high camp, then summit day, though arguably the really cold bit and where I was glad of the plastics is starting out from high camp (about 5,800m) to summit in the middle of the night. One member of our party turned back with numb feet at that point (and he had plastics on!).
Anyway, if the plastics are the only pair of crampon compatible footwear you take with you, then you could be wearing them over 3 days. I found them quite comfortable, especially if you loosened the top part around the ankle, on the easier terrain, especially on descent. The rest of the time you could get away with approach shoes/trainers (the porters certainly did), though beware - if the rainy season isn't quite over, you might be glad of waterproof but comfy walking boots with gaters.
Hope this helps
Damo