Preheat the oven to 220°c.
Add the onion and red pepper to a saucepan and spray with low calorie cooking spray. Cook over a medium heat for 5 minutes until softened.
Add the raw sliced sausage to the pan and cook for around 4 minutes until browned.
Stir the mushrooms into the pan and cook for 1 minute while stirring so they colour slightly.
Add the chopped tomatoes, butterbeans, Worcestershire sauce, balsamic, stock pot, garlic, paprika, and stir. Add salt and pepper to taste. Gently simmer, stirring occasionally for 10 minutes while you cook the potatoes. When the mix is done it will be rich and thick.
Bring a separate large saucepan of salted water to the boil. Add the potatoes and cook for 10-15 minutes until the potatoes are tender.
When the potatoes are done, drain the water and add the egg and marmite to the pan. Mash the potatoes until creamy. Season with salt and pepper to taste - though you may not need much salt as the marmite is already quite salty!
Add the sausage mix to a roasting or pie dish. Spoon the mash onto the top and drag a fork through it to rough up the surface.
Place into the oven for 15 minutes, until the top has turned golden. Serve hot with an accompaniment of your choice.