Put the butter, sugar, golden syrup and lemon rind in a saucepan and heat gently until the butter has melted. Remove from the heat and stir in the oats and sultanas. Mix well.
Spoon the mixture into a greased and parchment-lined 20 cm (8 inch) square shallow baking tin (see
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 20 minutes until turning golden. Leave to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely. Peel off the lining paper and cut into squares or fingers.