I recommend that you buy seed potatoes in January or February. The best size to buy is between 1 to 2 oz. However, the yield is not greatly affected by the size of potato that is planted, and all sizes offered by stores including the smallest, will give good results.
If you buy the potatoes before the recommended planting time, set them out in trays that contain 1" of dry peat with the end of the potato with the most eyes at the top, and keep in a cool, light, frost-free position until it is time to plant. Alternatively, you can buy potatoes at planting time and plant them straight away (after knocking off any long white shoots) but this will reduce the yield and give a later crop, especially for earlies.
Recommended planting times:
Earlies from late March, 12"between potatoes, 24" between rows and 5" deep
Maincrop from Mid-April, 15" between potatoes, 30"between rows and 5" deep.
Cover with soil and make a low ridge.
Protect shoots from frost if necessary by covering with protective material such as dry straw. When plant is approximately 6" high, draw loose up against stems to approximately 5" high. Increase the height of the ridge by further inch or two when the plant has grown sufficiently. Remember to water in dry weather.
First earlies June-July
Second Earlies July-August.
When flowers are fully open, examine potatoes. They will be ready when they are the size of hen's eggs. To lift, insert a fork well away from plant and ensure all tubers are lifted from the soil.
Main Crop August-October.
The above is taken from:Guide to Growing Seed Potatoes from Buckingham Garden Centre