Melt the butter in a saucepan over low heat. Add the leek and garlic and cook, stirring, for 2-3 minutes until softened. Season the prawns and add to the pan. Stir for 2 minutes or until just cooked, then remove the prawns with a slotted spoon and set aside.
Add flour to the pan and cook, stirring, for 1 minute. Add stock and wine and whisk to remove any lumps. Increase heat to medium then cook for 10 minutes or until liquid has reduced. Stir in cream, then simmer for a further 10 minutes or until mixture is thickened and reduced by half. Remove from the heat and allow to cool slightly. Add lemon juice, herbs and mustard, then return the prawns to the mixture. Season, then cool completely.
Using shells or ramekins as a guide, cut 12 pastry lids, leaving a 5mm border. Make light incisions in the pastry lids, following the shape of the shells. Divide filling among shells and top with pastry, pressing edges to seal. Chill for 15 minutes.
Preheat oven to 200°C.
Brush pastry lids with egg wash. Place pies on a tray and bake for 12-15 minutes until the pastry is cooked and golden.