Pre-heat the oven to 180°C, 160°C fan, 350°F.
Put the potatoes into a pan of salted water and bring to the boil. Cover and simmer for 15-20 minutes until tender. Meanwhile, heat the oil in a frying pan and add the bacon lardons, fry for a few minutes to cook through.
Add the leeks and continue to cook for 10 more minutes, until the leeks are tender and the bacon crispy.
Meanwhile, heat the butter in a non-stick pan, add the flour, stir to make a roux, then gradually add the milk, stirring and heating until thickened and smooth. Season and add most of the cheese.
Drain the potatoes, tip into an oven proof dish, pour on the leeks and most of the bacon.
Pour over the cheese sauce, sprinkle with the rest of the bacon and cheese and bake for 40 mins until warmed through and the top is golden.
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