Melt the margarine in a sizeable non-stick frying pan. Chop the spring onions finely and saute for a couple of minutes.
Slice mushrooms and add to the pan. Sauté for a few minutes until they colour.
Remove from heat. Sprinkle cornflour over the mushrooms. Add the Marmite and 300ml of the veggie stock. Cook another 5 minutes, till the mushrooms soften. If too dry, add more of the veggie stock.
Place mixture equally into 4 small ceramic pie dishes. Cut 4 circles of pastry for the pie tops, allowing room to rise. Brush on top with milk, inserting a hole in the top for steam to be released. Bake for 20 minutes at Gas Mark 6, removing when golden.