Bring a large pan of water to the boil, then add the potatoes and cook for 10 minutes until tender.
While the potatoes are cooking, heat a 20cm non-stick frying pan. Add the bacon and fry over a high heat for 3 minutes until browned. Add the sunflower oil and butter to the pan along with the leek and fry for 3 minutes until softened.
Drain the potatoes and add to the frying pan. Cook, stirring occasionally, for a further 3 minutes.
Preheat the grill to medium. Beat the eggs together in a bowl with 1 tablespoon water and season with freshly ground black pepper. Pour into the pan and cook over a medium heat for 5 minutes, tilting the pan occasionally to allow any runny egg to flow to the sides, until the base and sides are set and golden.
Transfer the pan to the grill, keeping the handle away from the heat, and cook for a further 2-3 minutes until the egg is just set in the centre. Mix the salad with the tomatoes and serve with wedges of the frittata.