Preheat oven to 200 degrees C. Parboil the potatoes and leave to cool a bit. Cook pasta until al dente then drain. Steam the leeks or boil them with the pasta (doesn't matter which - I boil them for quickness and then the pasta has a bit more flavour to it).
Heat up the tin of soup in a pan. Try it - if it needs any salt or pepper put some in. Put the pasta and leeks into a greased baking dish and mix in the tuna and heated soup till evenly mixed through.
Crumble the blue Stilton evenly over the pasta. Slice up the potatoes into reasonably thin slices (the thinner they are the crispier they'll be) and layer neatly covering all of the pasta.
Sprinkle evenly with cheddar and parmesan and put in the oven for 20 minutes or until the cheese on top is a light golden brown.