For the pastry, weigh out the plain flour in a bowl.
Over a medium heat, heat the butter, Crisp n Dry, water and salt. When it comes to a boil, take off the heat and pour into the flour.
Use a wooden spoon to combine the mixture until a dough forms.
Cover the bowl with a clean tea towel and leave to cool for an hour.
Lightly flour the work surface and roll out the dough into a rectangle. Fold the dough into thirds.
Roll it out again and fold into thirds again.
Put it in a dish, cover with cling film and put it in the fridge for 30 minutes.
Roll out two thirds of the dough to line a 21cm wide and 7.5cm deep pin tin. Brush the tin with oil. Place the pastry in the tin and trim the edges. Cover with cling film and put in the fridge for 30 minutes.
Now prepare the filling. Slice the potatoes thinly, and boil them for about 5 minutes. Drain and set aside.
Slice the leeks and cook for 5 mins with the oil, garlic, butter and a sprinkle of salt. Set aside to cool for 15 minutes.
In a bowl mix the double cream, lemon juice, cheddar, the eggs and herbs.
Mix the cooled leeks into the sauce.
Take the pie case out of the fridge and layer up the fillings, potato slices, then leek, potato, then leek, until it's full.
Roll out the rest of the pastry to make the lid. Beat the remaining egg and brush it around the edge to stick the lid to the base. Make a crimp pattern with your fingers and cut two slits with a knife in the middle. Brush beaten egg all over the top.
Bake on 180C/350F/Gas Mark 4 for an hour and a half. If the pastry on top starts to catch, cover it with foil.
Serve warm, any leftovers can be cooled to room temperature then stored in the fridge for up to 2 days. You can reheat the pie in the oven or microwave, make sure it is piping hot all the way through