To serve two, you need:
225g grated cheese
1 tbsp. butter
2 tsp Worcestershire sauce
1 tsp English mustard powder
2 tsp plain flour
4 tbsp. milk
bread - either sliced bread toasted on one side, or some crusty bread rolls or a baguette split in half.
Melt the butter in a small pan, add the flour and mix to make a roux. Add the cheese, Worcestershire sauce and mustard powder and stir until the cheese has melted.
Stir in the milk and you should have something resembling a thick paste.
Preheat the grill. Place the bread under the grill so the bottom starts to toast.
Remove from the grill and spread the cheese mixture on the other side.
Return to the grill and heat until the cheese is bubbling.
It's a good idea to place a baking tray lined with foil under the bread as I found mine got quite messy from melted cheese!