Pinched this from the Unwins (seed merchants) site
Growing in Containers
If you do not have space for a vegetable plot, potatoes can be successfully grown in containers provided they are at least 30cm (12 in) wide and deep with good drainage holes. The larger the container the more potatoes you will be able to grow. You can use garden soil mixed with multi-purpose compost as a growing medium, alternatively, the contents of growing bags give good results, with potatoes free from blemishes and skin problems. Place about 15cm (6in) of compost into the bottom of your container and plant your seed potatoes, 1 potato per 20cm (12in) pot, or more depending on the pot size, if you are using a really large container, for example a half barrel, then you can use 5 or 6 tubers. Cover the tubers with compost and keep adding more as they grow, until the container is full. For extra-early potatoes plant a tub or two in a frost-free greenhouse and watch how quickly they grow. When the leaves die back, harvest your crop, you will be amazed at how many potatoes are produced.
Feeding and watering
Potatoes are hungry feeders so use a good general purpose fertiliser such as Growmore. Particularly around flowering time, as this is when the tubers start to develop. Feeding is especially important when growing in a container, a tomato fertiliser is ideal for this. Watering is also very important and the best crop is obtained in soil that is uniformly moist. It is much better to water well occasionally, rather than little and often, as this encourages shallow rooting and plants will suffer in a dry spell.
Happy gardening!