I'm trying a couple of things to get an early crop of potatoes this year. The first is to simply buy a bagful of seed potatoes and let them sprout their roots on a window-sill. The second idea is to dig up odd plants that escaped last year's harvest, put them in large pots, and let them grow in the greenhouse. I've now done this, so let's hope that there's some winter growing action!
Early March update: The greenhouse potatoes are growing well, and I think that I should plant them out (very carefully of course, so the roots aren't disturbed) now that frosts are very likely to be over. The seed potatoes that I bought have just gone deep into the ground (around a foot down), and time will tell how their roots cope in the cold ground.