Pour the oil into a large non-stick frying pan and fry the onion gently over a low heat until it is very soft, this may take up to 15 mins. Add the garlic, grated carrot, chopped mushrooms and sunflower seeds. Season with salt and black pepper then add the other spices. Let it cook down for 5 mins to reduce some of the moisture from the vegetables. Add the Marmite and pearl barley and 200ml water. Bring to the boil, cover and cook for 30 mins or until all the liquid has been absorbed and the pearl barley is tender. Take the pan off the heat and tip the mixture out onto a large roasting tin so it can cool quickly. Once cold stir in the lentils, oats and vegetable suet.
When you want to cook the haggis spread it out on a large piece of baking parchment. Roughly shape it into a log then roll the paper up and twist the ends so it is completely sealed. Wrap up in muslin and tie the ends securely with string.
Heat a large pan of boiling water then slowly lower the haggis into it. Reduce the heat to a simmer and cook gently for 1hr. Unwrap the parcel and serve with neeps and tatties.