Preheat the grill to high. Melt the butter in a heavy-based pan over a low heat. Add the flour and cook for two minutes, stirring all the time. Add the milk, stirring to blend it smoothly. Cook for three to four minutes to thicken the sauce.
Take the pan off the heat and add the grated cheese, mustard and Worcestershire sauce, then mix thoroughly.
Add the white crabmeat and chives, and stir in.
 Lightly grill one side of the bread slices. Take out of the grill and spread the brown crabmeat over the untoasted side of the bread.
Top evenly with the white crab mix. Grill for four minutes until golden and crispy on top, and serve.