I have cut the potato haulms down just leaving stalks above the ground. I generally do this every year when blight starts to appear on the allotment site. In my experience it seems to attack the outdoor tomato plants first, which is a good early warning system. It is also the reason I don’t grow tomatoes on my allotment any more! This year I have not seen any signs of blight but it is getting so late on now I thought it best to do anyway.
Leaving a few inches of stalk above the ground works well as a potato marker and the potatoes can be left in the ground like this for a while longer. If you would like to know more about potato blight click here.