Peel potatoes and bring to boil in a large pan of salted water. Reduce temperature slightly, and cook for 12-15 mins or until cooked through.
Meanwhile, heat a little olive oil in a large frying pan over medium-low heat. Add onions, cook 4-5 mins until softened slightly. Add carrots and cook a further 10 mins. Stir through the peas and crumble in the veggie haggis. Mix together thoroughly, and pour into a suitable dish.
Mix the gravy powder mix with a little cold water into a smooth paste, and top up with 750ml hot water. Pour into dish over the haggis.
Drain potatoes in colander, and immediately add soya milk, marg and mustard to the pan (off the heat). Stir until marg melts, and add a pinch of salt and pepper. Return the potatoes to the pan and mash well.
Spoon or pipe the mash onto the haggis mix. Drizzle with a little extra olive oil. Bake in the oven at 180C/Gas Mark 5 for 30-40 mins. Serve with extra gravy.