In reply to llechwedd:
I have been having the same issue with a pair of Salewa Mountain Trainers. Had been considering buying a pair of stick on heel-grips. However, I came upon a pair of old Superfeet insoles, which I think were originally purchased for ski-boots, tried them in the shoes, instead of the supplied footbeds and have almost solved the problem. My heels still slip a tiny bit but it's marginal now, rather than feeling that my feet were going to come all the way out of the shoes.
The Superfeet seem to take up less volume than the original footbeds, so I have no idea why they work so well.