Here's my vegetarian haggis recipe. Inspired by this and this. I've only made this twice, so consider it a work in progress. But it came out well:
Serves 4--6 people. You can do most of it one day in advance, too.
Rinse the lentils and then boil them for 30-45 minutes. At the same time, boil the barley in a separate pan, until softened but with some bite. Drain both, and set them aside in separate bowls.
While they are boiling, prepare the rest. The onions and mushrooms should be finely chopped -- the size makes a difference to the texture of the haggis. Melt a good amount of butter/marge and fry the onion on a medium heat until well softened but not brown. Add the mushrooms and stir, frying for a couple more minutes before adding the spices. Give it one more stir.
Add the marmite and treacle to the stock and pour it into the pan. Add the lentils and the oats to the pan too. Bring it all to the boil, then turn the heat down and simmer gently (15 minutes?), until the mixture is thick and tender.
Grease a pudding basin. Stir the pearl barley and sunflower seeds into the mixture, then put it all into the basin. At this point, I let the whole thing cool and put it into the fridge overnight, for convenience.
Preheat an oven to 180C. Cover the pudding with foil and bake for 30 mins, then remove the foil and cook for another 15 mins. Leave for 5 mins, then turn out and serve, with mashed potatoes and swede (neeps).