Saute the leek (chopped) in a little oil for 2 mins, cover and cook for a further 3-4 mins or until tender. Transfer to a plate. Add the milk, butter and plain flour to the pan, bring to the boil, whisking all the time until a smooth sauce forms. Simmer for 1 min. Remove from the heat and stir in the mustard, the leeks and the chicken cut into bite sized pieces. Season to taste and gently warm through until the chicken is hot. Divide the mixture between 4-5 warm pancakes and serve with salad.