Parboil and add the tatties to the meat in the dish its cooking in. Meat can be cooked in advance but a true roast tattie - never! If the tatties are getting too cooked then you can put the in a dish at the bottom of the oven covered in foil or you could plate them and reheat them in the microwave, but get a good size spud and cut it in half and each half into quarters and you have 8 little potatoes to roast wedged in a baking dish alongside a joint of meat or turkey and they only take 40 minutes to cook this way.