The method I use is based on Heston’s in In Search of Perfection.

I use Maris Piper (floury), peeled and boiled in salted water using 10g salt per litre of water for 10 mins or until just starting to break apart. I set the oven to 175C with fan. I use olive oil to roast, pouring it into a roasting tray to a depth of about 2-3mm (don’t skimp) and heating the oil up for 10 mins before adding the parboiled potato. I turn the potatoes every 15 mins to get all the sides browned and crunchy. They take about an hour and can’t be rushed; the turning the temperature up results in them getting too dark without getting a thick, glassy crisp side.