Place the oil in a large frying pan and cook the bacon over a medium-high heat for 2–3 minutes, until golden. Reduce the heat slightly, add the garlic and leeks and cook for 4–5 minutes, stirring occasionally, until softened and golden. Add the mussels for the final 1–2 minutes, and stir until hot.
Meanwhile, cook the spaghetti in a large pan of salted boiling water for 3–4 minutes, or according to the packet instructions, until tender.
Put the eggs and cream into a bowl with plenty of black pepper and beat together.
Drain the pasta and toss quickly with the mussel mixture and creamy eggs until well coated. Heap into bowls and serve immediately.