Peel the potatoes and slice finely (1-2mm thick). Preheat the oven to 150ºC, gas mark 2-3. Rub the base and sides of a medium-sized gratin dish with the garlic. Layer the potatoes in the dish, seasoning them between each layer. Pour over enough cream to cover them completely. Bake in a slow oven for about 75 minutes until the cream has thickened, the top gratinated and the potatoes are tender.