Preheat oven to 180°C (160°C for convection oven). Line an 11 x 21 cm (base measurement) bread pan with baking paper.
Cook the pasta shells in a saucepan of boiling salted water for 10 minutes or until al dente. Drain well. Transfer to a large bowl.
Meanwhile, melt the butter in a large frying pan over medium-high heat. Add the leek and mushrooms to the pan and cook for 5 minutes or until softened, stirring often. Transfer to a bowl. Cool slightly.
Line the prepared bread pan with the bacon, slightly overlapping to cover and allowing the sides of the bacon to hang over the edge of the pan.
Add the fried leeks and mushrooms, eggs, cream, parmesan cheese and cheddar cheese and mixed herbs to the pasta. Season with salt, then mix well. Spoon the pasta mixture into the bread pan, pressing down well. Fold the overhanging bacon over the pasta. Bake for 40 minutes or until set. Set aside for 10 minutes to cool slightly. Carefully tip the pasta loaf out of the pan onto a serving plate. Sprinkle with the basil leaves and serve.