1. Preheat the oven to 180°C. Place the potatoes into a pan of cold, salted water. Bring to the boil and simmer for 10-15 minutes or until just tender. Drain well and set aside.
2. Heat the rapeseed oil over a medium heat in a large frying pan or wok. Add in the onions and carrots along with a good pinch of salt and fry for five minutes. Then add the leeks and mushrooms and continue to cook for a further 6-8 minutes, or until all the veg is soft. While the veg cooks combine the vegetable stock, water, soy sauce, yeast extract and Worcestershire sauce (if using) in a jug. Whisk to ensure that the veg stock is dissolved.
3. Stir the plain flour, garlic and thyme into the vegetable mix and cook for a further 1-2 minutes, then stir in enough of the liquid so that a thick sauce is created. Bring to a simmer then stir in the watercress and remove from the heat. Taste the mixture and add salt and pepper as needed. Pour into a large pie dish.
4. Roll out the shortcrust pastry to roughly the thickness of a pound coin. Cover the pie dish completely with the pastry then trim carefully. Crimp the edges with a fork and make a few small cuts in the centre to allow steam to escape while cooking.
5. Place the pie into the centre of the oven for 30-40 minutes, or until the pastry is cooked and golden brown. Remove from the oven and allow to rest for five minutes before serving.