Make the pastry using the rubbing in method. Rub together the flour, baking powder, margarine and sugar together to form breadcrumbs. Stir in the vanilla and lemon zest and enough water to bind together. Roll out the pastry and use to line a 9inch/23cm round fluted tin. Allow extra to hang over and prick the base with a fork. Place in the fridge to chill while you prepare the filling. Place the lemon juice and zest in a glass bowl over a pot of simmering water and warm for 3-4 minutes. Beat the egg yolks lightly in a bowl and then add to the warm lemon mixture, whisking constantly in a figure of 8 for 10 minutes over the simmering water till it thickens. Stir in the butter, remove off the heat and allow to cool down and thicken covered with cling film. Next, bake the pastry base blind in the oven at 200°C for 25 minutes. Remove from the oven and neatly cut off the excess overhanging pastry. Beat the lemon curd lightly and spoon into the tart case, spreading out evenly. Allow to cool down, then whip the cream to stiff peaks and pipe all over the top with a plain and star shaped nozzle. Decorate with berries, mint leaves and extra lemon zest.