Why do my potatoes have healthy foliage, nearly a metre high, but the potatoes are only marble size when dug?

Hi Nigel, it is more than likely your soil is too rich for your potatoes, excessive leaf growth is caused by too much nitrogen. Often this occurs if too much organic matter has been added to the soil such as compost, manure or nitrogen rich fertilisers. No real remedy will fix this problem for this year. You could try cutting back the tops by 50 % and adding some potato fertiliser to the soil, this may lead to your crop not being a total loss.