Put the leeks, butter and herbs in a pan, cover and cook over a low heat for about 20 mins until very soft, stirring occasionally. While the leeks are cooking, put the potatoes in a pan of cold water, bring to the boil and simmer for 4-5 mins until just cooked. Drain the potatoes and stir into the cooked leeks. Leave to cool, stir in the cheese and season with plenty of pepper and salt if you want. The filling can now be chilled for use the following day, if you like.