Heat butter and oil in a large heavy based pot over a medium heat. Add leek and celery. Cook, stirring, for 4 minutes or until soft but not brown. Add curry powder and cayenne and stir for 1 minute.
Add wine, water and brandy. Simmer on a medium heat for 3 minutes. Add seafood to pot and stir gently to coat with poaching liquid.
Cover and cook for 5 minutes or until fish is cooked and mussels have opened. Remove from the heat. Using a large mesh spoon, transfer seafood to a serving dish. Discard any mussels that haven’t opened. Cover dish to keep warm.
Whisk together cream and yolks in a large jug. Add a few tbsps of hot liquid from pot and stir quickly, then gradually add a few more tbsps until combined. Pour cream mixture into pot and stir constantly to prevent yolks from curdling.
Return pot to a medium-low heat and cook, stirring, until mixture thickens slightly, about 2-3 minutes. Season to taste. Pour sauce over seafood in dish and sprinkle with chervil. Serve with lemon wedges.