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.