Chop the vegetables into chunks, cutting denser ones (like carrots and swedes) smaller so they cook evenly.
Place them in a pot with just enough water to reach three-quarters of the way up the vegetables.
Add Marmite, rolled oats, salt, and pepper. Simmer until tender and most of the water is absorbed.
Transfer the mixture to a deep pie dish, sprinkle with parsley, and set aside.
To make the crust, combine flour, baking powder, and salt. Rub in the margarine, then mix in mashed potato until a dough forms. Add a little water if dry.
Roll out the dough to make a crust. Lay it over the filling, crimp the edges, and brush with milk.
Bake at 200°C (400°F) for about 30 minutes, or until golden brown.