Heat the oil in a large flameproof frying pan, add the leeks and potatoes and sauté for 8–10 minutes, stirring frequently, until golden and tender.
Add the bacon and fry for a further 4–5 minutes until cooked through.
Meanwhile, beat the eggs in a large bowl and add the cheese.
Season well.
Stir the potato mixture into the beaten eggs, then return to the pan and cook over a low heat for 8–10 minutes, making sure the bottom does not overcook.
Place the pan under a preheated hot grill and cook for a further 3–4 minutes until the tortilla is cooked through and golden.
Serve cut into wedges.