Use Maris Piper or King Edward.
You should have stored away oil that has fried any bacon, meat and herbs in a screw to jar, just for this. The objective when frying potatoes is to have a crisp deeply colored crust with a dense fluffy interior with a concentrated nutty/earthy potato flavour. To do this requites that water in the outer layers is gradually replaced by oil and the potatoe 'filling' dehydrates and condenses. This takes time - to avoid burning the potato starch and proteins while evaporating the water. So to accelerate the process cut the potatoes into pingpong ball sized pieces and boil vigorously for no more than 5 minutes in quite salty water - this heightens the flavour. Drain the potatoes and shake them until cool - to drive off as much steam (water) as possible, without damaging them too much.
Put new clean oil on the base of a large steep sided baking tin, add the potatoes and slowly put the flavoured oil over them so they are saturated. Cook them in the centre of an oven at 190 for 20 minutes, turn and shake, baste with more oil, repeat after another 20 minutes and put a lot of fresh rosmary on the pan. This is to fill the kitchen with a sillage of rosemary which should just taint the oil.
Keep checking for the potoe surface to darken to a mottled red/gold, then remove from the oven and tilt the pan to drain as much oil as possible, remove with a slotted spoon and place in a glass bowl so you can see the colours glisten. If you have meat fat add this to the pan towards the end of cooking, but it is not strictly necessary. You can decorate with more fresh rosemary and the heat will send a cloud of rosemary fumes into the dining table air.
If you are roasting with meat bear in mind that your are tryng to evaporate the water so while the potatoes are cooking make sure the meat is covered otherwise it will increase the humidity in the oven too much for effective 'crisping'.
