If you are using an electric fondue pot, turn it on to medium; otherwise use a double boiler to heat up this mixture. Place beer, garlic, dry mustard, and Worcestershire in the pot and combine well. Shred or cube the cheeses and toss with flour, coating the cheese well. (The flour will help thicken the sauce.) When the beer mixture is warm, add one-third of the cheese and whisk very well. Once the cheese has been incorporated well, add half of the remaining cheese and whisk in very well. Add the remaining cheese and whisk until nice and smooth.