Pre-heat the oven to 200C/180C Fan/400F / Gas 6. Put the potatoes in cold salted water, bring to the boil then reduce the heat and simmer for 10 to 15 minutes until tender. Drain and mash with butter, milk and the grated cheeses. Melt the butter in a saucepan and cook the leek and shallot gently for 8 to 10 minutes until soft but not coloured. Stir in the flour and cook for one minute, then stir in the stock a little at a time, to make a smooth sauce (you may find it easier to do this off the heat).