Peel the marrow, cut in half lengthways and scoop out the seeds. Bring a large pan of lightly salted water to the boil, add the marrow (cut in half if the pan is not big enough) and simmer for 4-5 minutes. Drain and arrange cavity side up in a lightly oiled ovenproof dish. Preheat the oven to 190 C, 375 F, Gas Mark 5. Melt the butter in a saucepan, stir in the flour and cook for 1 minute, stirring occasionally. Gradually stir in the milk, bring to the boil and simmer for 2-3 minutes until thickened and smooth. Remove from the heat and stir in most of the cheese, most of the herbs, the chicken and seasoning to taste. Spoon into the marrow cavities. Mix together the breadcrumbs, remaining herbs and seasoning to taste. Sprinkle over the marrow filling and bake in the preheated oven for 25 minutes until heated through and golden. For an extra crunchy top, place under a preheated hot grill for 1 minute before serving. NOTES : A delicious creamy cheese sauce with chicken fills the marrow which has a crunchy herb topping.