Cut the bread into large bite-sized chunks and put in a serving dish. Peel the garlic, cut it in half and crush or rub it well around the base and sides of the fondue pot, to leave a good garlicky flavour. Discard the garlic pieces.
Grate the cheeses. Put the cornflour or flour into a cup and mix to a smooth paste with 2-3 tbsp of the chosen wine, cider or apple juice. Put the remaining liquid into the fondue pot, add the lemon juice and heat gently over a low heat, gradually stirring in the grated cheese and mustard with a wooden spoon, stirring until all the cheese has melted. Remove from the heat, stir the cornflour mixture again, then mix it into the fondue. Return the pot to the heat and cook the fondue, stirring all the time until it is thick, smooth and just bubbling - the heat must remain turned down low all the time.
Season well with black pepper and carefully carry the fondue pot to the serving table and place over a burner if you have one, or on a hotplate or thick table mat. Always use the correct fuel recommended for your burner (generally methylated spirit) and put the burner in place before lighting. Never carry a lighted burner from kitchen to table.
Eat the fondue at once, spearing the bread cubes and dipping them into the hot cheese - it's a more filling meal than you might expect. My family like fondue served with a few crudites (sticks of cucumber, carrot, celery, pepper strips, cauliflower florets, radishes, sliced mushrooms, etc.) or a crunchy salad, to offset the richness of the cheese. A bottle of dry white wine makes a good accompaniment too!