Heat the oven to 200C/180C fan/gas 6 and oil a 22cm pie dish. Cook the potatoes in a large pan of boiling salted water over a high heat until just tender, about 12-15 mins. Once the potatoes are cooked, drain, then tip into the pie dish and roughly break up using a potato masher until they cover the base of the dish and up the side, forming a crust. Brush over more oil, then bake for 10-15 mins until beginning to turn golden.