Preheat the oven to 375°F (190°C or 170°C for a fan oven).
Put the potato pieces in a saucepan and cover them with cold water.
With the lid on, bring the potatoes to the boil.
As soon as the potatoes come to the boil, set the timer for 5 minutes.
Meanwhile, put the oil in a roasting pan or on a baking sheet with sides and heat in the oven.
When the timer goes off, drain the potatoes well.
Put the lid back on the saucepan and shake the potatoes gently to rough up the edges.
Remove the roasting pan or baking sheet from the oven carefully (the oil will be very hot) and set it on a heat proof surface.
Using tongs, transfer the potatoes to the roasting pan or baking sheet, turning them carefully in the oil.
Cook the potatoes for 20 minutes before turning carefully using tongs.
Cook for 15 minutes more or until lightly brown and crispy.