Preheat the oven to 220°C. Put the potatoes in a saucepan and cover with water, add a pinch of salt and bring to the boil. Cook for 15-20 minutes until soft.
White the potatoes are cooking, put the sausage pieces, onion and celery in a frying pan and spray with low calorie cooking spray. Cook over a medium heat for 5 minutes, until the sausage has started to brown and the onion starts to soften. Add the stock, vinegar, Henderson’s relish, garlic granules, thyme and mustard powder, then reduce the heat and simmer for 5-10 minutes until the onions are soft and glossy. Remove from the heat and stir in the tomato puree. The mix should be coated in a thick gravy but add a splash of water to loosen it if you need to. Stir through the peas and set aside while you prepare the mash.
When the potatoes are done, drain the water and mash the potatoes until creamy. Add the egg and mustard powder and mix through. Season with salt and pepper.
Spread the sausage mix in a roasting dish or pie dish. Spoon the mash on top and use the back of the spoon to smooth it out. Place in the preheated oven and bake for 15 minutes, or until the top is golden and the pie is piping hot throughout.