Preheat oven to Gas Mark 4.

Parboil the potato slices for 3 minutes, drain and rinse under cold water. Layer one third over the base of a large butter ovenproof dish.

Melt half the butter and gently fry the leeks for 4-5 minutes until soft. Spoon leeks over potatoes.

Melt the remaining butter in the pan. Add the flour and cook for 1 minute. Remove from heat and blend in the milk and cream gradually, bring to the boil, stirring, then simmer for 1 minute.

Pour half the sauce over the leeks and potatoes, arrange the remaining potatoes in a layer on top. Spoon the remaining sauce over the potatoes, sprinkle over cheese and bake for 40 minutes.