Heat the butter in a pan. Add the flour, cook briefly; the flour must not take on any colour. Pour in all of the beer and milk at once, bring to the boil while stirring, reduce the heat and simmer for approx. 5 mins. Stir in the cheese and Worcestershire sauce, continue stirring until the cheese has melted.
With the lid down, grill the slices of bread over/on a medium heat (approx. 200 °C for approx. 2 mins. on one side. Remove, spread mustard over the toasted side of the bread, spoon over the cheese sauce, sprinkle with paprika. Cover and grill the slices of bread again for approx. 10 mins. Remove, scatter parsley over the cheese toasties.