In reply to Lancs Lad:
I suspect you might need a wee bit more than that.
I was supposed to be going up something called Nevado Huarancante down near Arequipa between Christmas and New Year.
It was cancelled due to combination of:
a) Weather will probably be rubbish;
b) Can't take my stuff as I have to take out tonnes of shortbread/whisky etc to my wife's family.
Anyway, I was speaking a few weeks ago to her cousin's boyfriend who is Peruvian and who has done a fair amount of stuff in the Cordillera Blanca. He reckoned that for 5300m in the south of Peru I would be able to get away with my Mantas and a liner sock/thick sock. That's mainly rock/scree though with not a lot of snow. I was up to 5100m in the Cordillera Blanca with a pair of Hi-Tec V-Lite boots and a trekking sock - again, no snow though.
He uses Vasque Super Alpinistas in the Cordillera Blanca. He's from Lima though and basically they moan it's cold as soon as it drops below 20C ;0