Grease a shallow 12x8-inch cake tin, line the base and sides with foil and grease the foil. Mix together the biscuits, melted butter and coconut. Press the mixture evenly into the tin and smooth the surface.
Combine the condensed milk, extra butter, sugar and golden syrup in a small pan. Stir over low heat for 15 minutes, or until the sugar has dissolved and the caramel is smooth, thick and lightly browned. Remove from the heat and leave to cool slightly. Pour the caramel over the biscuit base and smooth the surface.
Put the milk chocolate melts and oil in a heatproof bowl. Half fill a saucepan with water and bring it to a boil. Remove from the heat and place the bowl of chocolate over the pan, making sure the base of the bowl is not sitting in the water. Stir occasionally until the chocolate has melted and the mixture is smooth. Spread the chocolate mixture over the caramel. Allow to partially set before marking into 24 triangles. Refrigerate until firm.
Storage: The slice can be stored in an airtight container in a cool place for up to 2 days.
Variation: Use dark chocolate melts in place of milk chocolate.