For me Christmas potatoes means a combination of well stored maincrop potatoes and freshly dug new ones. Now it is possible to buy seed potatoes direct from the horticultural suppliers specially kept back for planting in August. First earlies such as ‘Maris Peer’ are an excellent choice. First early potatoes take 12 weeks from first growing to cropping, which if planted in August should give you potatoes in mid November, but if you account for a lack of daylight and the general drop in temperature, even in an unheated polytunnel, you will get a crop in December, and into January.