You can have them but you have to syn them or use them as a Healthy Extra (B choice). If you use it as a HEXB you have to leave the skin on and you get 227g of baking potatoes or 198g new potatoes.
Otherwise syns are ad follows:
Potatoes, all varieties, flesh & skin, raw, 100g 2½
New potatoes, in skins, raw/cooked without fat, 100g 3½
Potatoes, all varieties, flesh only, raw/cooked without fat, 100g 4