Heat the oven to 220C/200C fan/gas 6. Bring a large pan of salted water to the boil, then add the potatoes, reduce the heat and simmer for 8-10 mins. Drain, then leave the potatoes to steam-dry in the colander for a few minutes. Shake to slightly roughen the edges.

Meanwhile, drizzle the oil over a large baking tray and put in the oven to heat up for 10 mins. Once hot, tip the potatoes onto the tray with the butter and garlic. Sift over the English mustard powder and toss everything together so the potatoes are evenly coated. Roast for 45 mins, turning halfway through cooking for really crispy potatoes. Season with a pinch of sea salt flakes to serve.