Preheat oven to 190c
In a large heavy-based pan, sweat down the leek, carrots, celery, thyme sprigs, star anise and cloves for 20 minutes over a low heat until soft and colourless.
Remove the haggis from its skin and crumble into the dish, turn up the heat and cook for 5 minutes. Stir through the tomato puree and allow to cook out for another 5 minutes.
Scrape the bottom of the dish while adding chicken stock, Marmite, brown sauce, mustard, Worcestershire sauce and season. Allow to simmer gently for 30 minutes until the sauce has thickened nicely, stirring periodically so it doesn’t catch underneath.
Taste again to ensure it's well-seasoned and balanced.
In the meantime, boil the swede and potatoes until tender. Drain the water, then add the butter, yoghurt and half the warmed milk and blitz with a stick blender until smooth but still thick.
If it’s a little too thick, add the remainder of the milk – more if you wish. Season very generously with plenty of black pepper.
Assemble the pie in a heatproof dish – layering the buttery swede over the peppery haggis. Cook in the middle shelf with a baking tray under the dish to catch any drips for approx. 30 minutes or until the top is golden.