Preheat the grill. Arrange the sausages on the grill rack and cook for 10-15 minutes, turning often until browned.
Meanwhile, heat the oil in a frying pan and cook the onion for 4-5 minutes, until browned. Add the mushrooms and cook, stirring often, for 3-4 minutes.
Dissolve the gravy in 200ml of boiling water. Stir in the mustard, then add to the onion mixture. Slice the sausages and add them to the pan. Transfer to an ovenproof baking dish and leave to cool a little while making the topping.
Cook the macaroni in lightly salted boiling water for 8-10 minutes. Meanwhile put in the milk, spread and flour into a non-stick saucepan. Heat, stirring constantly with a whisk, to make a smooth sauce. Add the mustard and season with pepper. Stir in the Cheddar cheese and allow it to melt. Drain the macaroni thoroughly and add it to the cheese sauce. Spoon on top of the sausage mixture and sprinkle the Red Leicester cheese onto the surface.
Preheat the over to 200°C, fan oven 180°C, Gas Mark 6. Bake for 20 minutes, then grill the top for 3-4 minutes, until golden brown. Serve with green vegetables.