I dug up one of each variety of the potatoes - shown left. There were a reasonable number of spuds, but they were quite small. Unfortunately there are unmistakable signs of Potato Blight on both. One of the King Edwards was squashy and disgusting, but the rest were unblemished. All the Epicure had the beginnings of rot on the surface, but were OK in the middle. Once I knew what to look for, I went over all the plants and snipped off blighted leaves and stalks, hoping to reduce the number of spores drifting down onto the soil. I hope this will slow the effects of the disease on the crop.
I think I'll harvest them as needed now - the potatoes won't be full-size, but I won't run the risk of the whole lot being ruined. I hope.