Crush the garlic to a paste using a pestle and mortar, then mash in the anchovies – you can also do this with the edge of a knife on a chopping board. Spoon into a bowl and finely grate in the parmesan, then stir in the mayonnaise and vinegar. The dressing should be the consistency of yogurt – if it is thicker, stir in a few drops of water to loosen it. Will keep chilled for three hours.