To make the pastry rub the butter into the flour with a generous pinch of salt and mix with fingertips until mixture resembles fine breadcrumbs. Stir in egg and water mixture, bring together to form a dough and knead very lightly. Divide the dough into 2/3'ds for base and 1/3'ds for the top. Roll into balls, flatten slightly, wrap each ball in cling film and chill for 20 mins. If preparing dish early in day (or day before) you can prepare to this point and store filling and dough in fridge until next day or when ready to assemble. Remember to bring dough to room temperature before rolling.