Place the butter in a medium saucepan and heat gently until melted. Stir in the sugar, then add the chocolate and stir over very low heat until melted and the mixture is smooth.
Add the raisins to the pan, then take 3-4 biscuits at a time and break them between your fingers into small chunks as you add them to the pan. Stir well until all the biscuits are coated in chocolate.
Line a 20cm shallow square tin with baking paper, then add the mixture and press it down as flat and smooth as you can. Leave to set in the fridge for a couple of hours (or freeze for 20 minutes)
To make the topping, melt the milk chocolate with the oil in a bowl over a pan of gently simmering water (or in the microwave in 30-second bursts) then spread over the tiffin and leave to set again before cutting into squares.
The oil in the milk chocolate makes it easier to cut – but you only need a little!