Preheat the oven to Gas Mark 6, 200°C, fan 180°C. Heat the oil in a roasting tin in the oven. Bring a pan of salted water to the boil. Add the potatoes, bring back to the boil and cook for 5 minutes.
Drain the potatoes, let them steam for a few minutes, then return to the pan and shake vigorously. Put them into the hot fat and turn carefully to coat. Season, then roast for 40 minutes.
Meanwhile, pick the leaves from the thyme and roughly chop the rosemary needles. Scatter the herbs over the potatoes, toss well, then roast for 10 minutes more or until very crisp and golden.
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