Place the first three ingredients in a food-processor and add the butter in knobs. Whizz until combined, then, with the machine running, add 1 tablespoon of cold water and combine briefly until you can bring the dough together with your hands.
Tip onto a large sheet of clingfilm and roll into a log, about 18cm/7in long. Wrap tightly in the wrap and chill for an hour or so. Then cut into