Sift the flour and icing sugar into a large bowl and add the soya margarine.
Use your fingertips to rub the fat into the flour until you have a mixture that resembles coarse breadcrumbs with no large lumps of margarine remaining. Try to work quickly so that it doesn't become greasy.
Bring the mixture together to form a dough, adding extra flour if the mixture is too wet, or some water if too dry, a teaspoon at a time.
Wrap the dough in cling-film and chill for 10-15 minutes before using.