Tip the raisins into a bowl and pour over the sherry, then leave to soak for 1 hour.
Heat the milk, cream and vanilla extract in a saucepan until almost boiling. Meanwhile, beat together the egg yolks and sugar in a heatproof bowl until pale and thickened. Stir in a spoonful of the hot cream mixture, then pour the egg mixture into the pan and cook over a low heat, stirring frequently, until it coats the back of a wooden spoon, making sure it doesn't boil. Remove the pan from the heat, cover the surface of the custard with clingfilm and leave to cool.
Place the custard in an ice-cream maker and churn until frozen following the manufacturer's instructions, adding the raisin mixture just before the ice cream is ready. Alternatively, freeze the custard in a shallow freezerproof container until almost solid, then beat until smooth and return to the freezer. Repeat this 2 more times, stirring in the sherry and raisin mixture after the final beating. Freeze until solid.
Serve scoops of ice cream drizzled with extra sherry or Caramel, Sherry & Raisin Sauce (see below), if liked.