Add the potatoes to a pot of cold water and cover with a lid. Place over a high heat and bring to the boil. Reduce the heat and simmer until the potatoes are tender when pierced with the tip of a knife.
Melt the butter in a pot over a medium heat. Add the sprouts and leek and cook for 8-10 minutes until soft, stirring often. Season with salt and pepper.
Meanwhile, heat a large non-stick frying pan until hot, add the chopped bacon and fry over a medium heat for about 5 minutes until crispy and golden.
When the potatoes are cooked, remove from the heat, drain in a colander, then return to the pot and add the milk.
Using a potato masher, mash the potatoes until smooth and creamy. You may want to add a little extra milk to reach your desired consistency,
Add the buttery sprouts and leeks and stir until evenly combined. Taste and add extra seasoning if needed, then serve.