Preheat the oven to 200C/180C Fan/Gas 6.
Meanwhile, to make the filling, put the fish stock, vermouth and saffron in a wide saucepan and heat gently. Add the leek, fennel, langoustines, smoked salmon, haddock, monkfish and scallops and poach gently for 5 minutes, or until just cooked. Discard the vegetables and place the seafood in a large bowl. Peel the langoustines, keeping a couple of heads. Reserve the poaching liquid.
Melt the butter in a saucepan and gradually add the flour, stirring, until you have a paste. Pour in the Prosecco and 100ml/3½fl oz of the fish poaching stock , whisking constantly, until you have a smooth sauce. Add the cream and chives and season with salt and pepper.
Pour the sauce into the bowl with the seafood and mix well. Spoon the filling into a pie dish and set aside to cool.
Roll out the pastry to the thickness of a pound coin. Brush the edges of the pie dish with the beaten egg then lay the pastry over the top and press down around the edges, crimping to seal. Trim any excess pastry, then brush with egg and pierce a hole in the middle. Garnish with the langoustine heads and bake for 30-40 minutes, or until piping hot and golden-brown. Cover with kitchen foil if the pastry is browning too quickly.