If you maintain non-listed potatoes (there are plenty of listed heritage varieties) and you don't live in a blight free area (North Scotland) then you need to be very careful.
Heritage varieties aren't "expensive". They have no value as you are not allowed to sell the seed, the most you can do is purchase microplants.
To maintain non-listed varieties, keep the potatoes growing as far away from others as you can, keep a very watchful eye out for blight. Lift early to avoid blight, but if you have the slightest hint of it, cut off the tops immediately, leave in the ground for 3 weeks and then harvest.
Inspect tubers frequently for signs of blight and if there is the slightest hint of it, dispose of them. There are lots of enthusiasts who will happily replace your stock for you.
Phil