Preheat the oven to 350. Cook the potatoes in plenty of lightly salted boiling water for 3 minutes until slightly softened, then drain. Cut the leeks into 1/2 inch lengths and blanch them in boiling water for 1 minute until softened, then drain.
Turn half the potatoes into a shallow ovenproof dish and spread them out over the edge. Cover with 2/3 of the leeks, then add the remaining potatoes. Tuck the slices of cheese and the remaining leeks in among the top layer of the potatoes. Season and pour the cream over.
Bake for 1 hour until tender and golden. Cover with foil if the top starts to overbrown before the potatoes are tender.