Bring a large saucepan of water to the boil and add the pasta. Bring the water back to the boil, cook for 5 minutes and then add the leek and broccoli and cook for a further 5 minutes.
Preheat the grill to hot and cook the bacon, if using.
Pour the milk into a saucepan and, over a medium heat, whisk in the flour. Bring to the boil and keep whisking to make a smooth sauce. Simmer for four minutes and then add the mustard and half the Cheddar cheese, season well.
Drain the pasta and vegetables, tip back into the saucepan. Add the cheese sauce and stir gently until all of pasta is coated. Spoon mixture into the ovenproof dish and sprinkle with the rest of the Cheddar cheese.
Arrange the tomato wedges on top and scatter bacon if using. Sprinkle with parmesan and put dish under a hot grill to brown.