In a large bowl rub together the two flours and dairy free margarine until the mixture is the consistency of fine breadcrumbs.
420 g plain flour, 80 g strong white bread flour, 100 g dairy free margarine
In a saucepan over a low heat, add the water, shortening and salt. Heat gently until the shortening has melted but don’t let the mixture boil.
200 ml water, 120 g vegetable shortening, 2 tsp salt
Once the shortening has melted into the water, bring the mixture to the boil. Once the liquid is in a rolling boil (about 1 minute), take it off the heat and pour it over the flour mixture straight away.
Mix the flour and liquid together with a spoon and once it is cool enough to handle, knead it gently for two minutes to bring the mixture together fully.
Lightly flour a surface and roll the dough out to 1/4-1/2 cm (1/4 - 1/8") thick. You want to work relatively quickly because the dough is much easier to roll out and handle when it is still warm.
Select a baking tin - ideally a non-stick springform cake tin (20cm / 8") for a large pie or a large muffin tin with removable bases for small pies. As long as you are using a good non-stick tin with a removable base you should not need to line the tin with parchment paper.
Cut out the pastry to fit your baking tin and gently shape it to the tin, leaving a 1-2 cm (1/2-1") overhang around the edge to join the lid to later on. Hot water crust pastry is very forgiving so you can patch, press and stretch it as necessary to fit your tin.
Cut out a lid with a small steam hole in the middle and place it on a tray. Put the lid in the fridge along with your pie case for 30 minutes to chill and firm up.