Place bread and vinegar in a small bowl and leave for 15 minutes, then squeeze out excess liquid. Place garlic on a work surface, sprinkle with ½ tsp sea salt and, using the back of a large knife, crush garlic until a smooth paste forms. Place soaked bread, garlic, anchovies and tarragon in a bowl, then, using a wooden spoon, stir to break up soaked bread until well combined. Gradually add oil, drop by drop at first, until mixture is emulsified, then in a thin stream until thick, then season to taste with freshly ground black pepper. Cover closely with plastic wrap and refrigerate until needed.