Rub together the fat and the flour (either by hand or using a food processor on the pulse setting) until the mixture resembles fine breadcrumbs. Add the caster sugar and mix to ensure it is distributed evenly. Next add the water a tbsp at a time. Using a spatula or butter knife mix in the water. When the flour has begun to clump together use your hands to bring the mix into a ball.
Place in a bowl covered with cling film and pop into the fridge to rest for half an hour.
While the pastry is resting take off the outer layer of skin from the rhubarb and chop into pieces roughly 1 inch long. Places these in a large heavy pan with 4 tbsp of water and the ginger grated finely. Let the rhubarb stew for 5 mins before giving it a stir. Stew the fruit for another 5 mins or until it has started to become tender. Drain the rhubarb retaining the liquid for the syrupy sauce.
Allow the rhubarb to cool whilst adding the remaining sugar to the pan with the rhubarb juices. Avoid stirring the mix and let the sugar dissolve. Gently let the sugar syrup bubble until it has gone golden and thick (roughly 5 mins on a low heat). Take off the heat immediately once the desired consistently has formed.
Roll out 2/3rds of the pastry for your bottom and 1/3 for the top. Both should be roughly 2.5mm thick. You should have a little excess for decoration. (I like hearts or stars!)
Ease the base into your pie dish before pouring in the cooled rhubarb and the syrup. Place the lid over the top and seal firmly around the edges. Using a sharp knife create a cross in the middle of the pie so that the heat can escape.
Add any decorations you like before adding a wash of milk to help the pastry brown.
Place in an oven on gas mark 4 for 35-45mins or until the filling is hot and pastry cooked.
Serve immediately with vanilla ice-cream, custard or natural yogurt.
NB. This makes enough pastry and filling to fill a pie dish 26cmx5.7cm