Peel the potatoes (700g) and chop so that they're roughly even in size (about 5cm pieces).
Put the potatoes into a medium saucepan and cover with cold water, season with salt.
Cover the pan and bring to the boil, once boiling continue to cook for a further 10 minutes.
While the potatoes are boiling, pre-heat the oven to 220ºC/200ºC fan. Put the olive oil (4 tsp) into a baking tray and put the tray into the oven to heat (the tray needs to be large enough that all the potatoes have a little space - otherwise they won't get crispy).
Once the potatoes have been boiling for 10 minutes, drain the water. Give the potatoes a gentle shake so that the edges get a little roughed up (but not too much or they'll fall apart). Remove the tray of hot oil from the oven and carefully put the potatoes into the tray of hot oil. Season with salt and return the tray to the oven for 25 minutes.
Remove the tray from the oven, turn the potatoes. Season the other side with salt and return to the oven for a further 20 minutes.
Remove from the oven and serve.