Heat the oven to 220C/200C fan/gas 7. Peel the potatoes and chop into medium-sized chips. Rinse under cold running water, then tip into a large pan, cover with cold salted water, add the vinegar and bring to a simmer over a medium heat. Stir once, then continue to simmer for about 6-8 mins until the chips are just cooked through but not breaking apart. Drain well.

Add the oil to a shallow roasting tin and put in the oven to heat for 5 mins. Toss the chips in the cornflour, then carefully tip into the hot oil and arrange in a single layer using tongs. Drizzle with a little more oil to coat, then roast for 20 mins.

Carefully turn the chips over in the oil using a spatula, then return to the oven for another 20 mins until golden brown.