Preheat oven to 180C (Fan 160C).
Boil potatoes for 15 - 20 minutes. Mash with milk and butter.
Boil leeks for 10 minutes.
Places chunks of corned beef in bowl with tomato purée and Worcestershire sauce. Mix lightly.
Pour baked beans into ovenproof dish. Spread the corned beef mixture over the top. Cover with mashed potatoes and sprinkle with cheese.
Bake for 30 - 40 minutes.