Boil the potatoes and garlic for 20 minutes, drain well, then return to the pan and place on a low heat. Shake the pan to steam off the excess moisture for a few minutes. Remove from the heat and mash the potatoes and garlic until very smooth (a potato ricer produces the best results), then add the butter, milk and seasoning. Beat well. Add the cheese and beat vigorously for 5 minutes until all the cheese has melted to form a rich, elastic mixture. Serve at once.