Push a wooden cocktail stick into each apple core, making sure they are secure. Put the sugar and water in a large heavy-based saucepan, with a sugar thermometer attached, and heat gently until dissolved.
Add the remaining ingredients and bring to the boil.
Boil until the temperature reaches the soft crack stage 143 C (290 F), when a little of the syrup dropped into cold water separates into hard but not brittle threads.
Brush down the sides of the pan occasionally with a pastry brush dipped in cold water.
Do not stir.
Dip the apples into the toffee, twirl around for a few seconds to allow excess toffee to drip off, then leave to cool and set on a buttered baking sheet or waxed paper.