For tzatziki, coarsely grate cucumbers and place in a colander, stand for 15 minutes, weighted with a plate, to drain out excess liquid. Place grated cucumber in muslin and squeeze out any remaining liquid.
Place garlic and a little sea salt in a mortar and, using a pestle, pound to a paste.
Place cucumber, garlic paste, Yoghurt and vinegar into a food processor, season with freshly ground black pepper and process until smooth, slowly adding the olive oil with motor running. Transfer to a bowl, cover and refrigerate until ready to use.