Pour 1 litre (1¾ pints) water into a shallow pan - it should be about three quarters full. Add the vinegar, but no salt, and bring it to the boil. One by one, break four eggs cleanly on the edge of the pan, open just above the surface of the water and slide them smoothly into the pan. Place over a gentle heat so that the water does no more than tremble and cook for 2½ mi