Leave small and medium potatoes whole, but halve large potatoes. Tip the potatoes into a large pan of cold salted water and bring to the boil. Boil for 6 mins exactly, then drain in a colander and leave to steam-dry for a few mins. Toss in the colander to rough up the sides.
Transfer to a large bowl. Shake over the flour, some salt and a little pepper, and toss well to combine. Spoon over 100g of the fat and toss to coat evenly. Lay the potatoes on a tray lined with baking parchment and freeze. Once frozen, the potatoes can be transferred to a freezer bag for easy storage. Can be frozen for up to 3 months.
When ready to cook, heat oven to 200C/180C fan/gas 6. Pour remaining fat and the oil into a baking tray and put in the oven. When the fat is sizzling, carefully add the potatoes, then roast for 25 mins. Remove tray, then increase oven to 220C/200C fan/gas 7. Turn over the potatoes and return to the oven for a further 40 mins until golden and crisp.