Preheat the oven to 200°C, 400°F, Gas Mark 6 and put in a baking sheet to warm. Rub the butter into the flour with the salt until you have coarse breadcrumbs. Add water a little at a time using a round-bladed knife to mix until the mixture just comes together. Form into a ball, cover with clingfilm and refrigerate for 20-30 minutes. Meanwhile heat the olive oil in a pan and gently fry the bacon until the fat has rendered and the bacon is lightly gold. Remove from the pan with a slotted spoon and lay on kitchen paper to drain. Add the leeks to the olive oil and bacon fat and cook very gently until they are softened and sweet. Roll out the pastry and press it gently into a lightly greased flan tin. Prick all over with a fork and bake in the oven on the baking sheet for 20 minutes until pale gold. Spread the leeks and bacon evenly over the pastry base. Whisk together the eggs, egg yolk, cream and crème fraîche and seasoning then pour over the vegetables. Sprinkle the top with Parmesan and bake in the oven for 35 minutes or until the centre is just set and the filling golden. Leave to cool and serve. This recipe is very indulgent, so shouldn't be eaten too often. But it's perfect for those special occasions!