Mix together the flour, salt and cayenne
in a bowl, then rub in the butter or margarine until the mixture
resembles breadcrumbs. Stir in the cheese, then mix in the egg yolk
and enough of the water to make a firm dough. Turn out on a lightly
floured surface and knead gently until blended. Wrap in clingfilm
(plastic wrap) and chill for 30 minutes before use.