Put the mustard, vinegar and water in the liquidiser with a good grinding of black pepper and about half a tsp sea salt. Mix very thoroughly (at least 20 seconds). Then, with the machine running, start to add a dribble of oil, stopping the dribbling every now and then but leaving the machine running to make sure the mixture is thoroughly emulsified (just like making mayonnaise). Towards the end, the dribble can be increased to a thin stream. You should end up with an emulsion which is thicker than double cream but a bit thinner than a good mayonnaise.