Preheat oven to 200C/400F/Gas 6
Blanch the peeled and chopped potatoes in boiling water for 2-4 mins, then drain well. Cut the leeks into chunks and in a slightly greased oven proof dish, layer the potatoes and leeks alternatively. Then sprinkle with the chives.
To make the sauce, melt the butter in a saucepan and add the flour. Cook until it looks sandy in texture then add the milk and cook for 5-10 mins until thickened. Add nearly all the cheese, save some to sprinkle on top.
When the cheese has melted pour over the potatoes and leeks then sprinkle with the remaining cheese.
Bake in the oven for 25-30 mins until golden brown.
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