Lightly grease and line a non-stick 30 x 20-cm brownie pan.
In a saucepan, place the butter, sugar and golden syrup. Warm on a low heat until melted and stir to mix together. In a mixing bowl, grate 100g of the milk chocolate, then add the dried fruit and nuts, strawberries and crushed digestive biscuits. Pour in the melted butter, sugar and syrup mixture. Fold together until thoroughly mixed, then spoon into the lined brownie pan. Smooth the tiffin base to make it level, then place in the refrigerator for 15 minutes.
Melt the white chocolate by breaking it up and heating three-quarters of it in a microwave on high in a microwaveable bowl. Use the microwave in 10-second bursts, stirring the chocolate in-between until it is all melted. Now, add the remaining one-quarter and mix together to form a smooth, just-melted chocolate.
Remove the tiffin base from the refrigerator and pour the white chocolate over the top. Tilt the pan until the topping covers the tiffin base and is smooth and even.
Now melt the remaining milk chocolate in the same way (melting three-quarters of it, then adding the final one-quarter at the end). Pour the milk chocolate over the white chocolate in thin lines. Use a cocktail stick/toothpick to drag the milk chocolate over the surface to form patterns. Return to the refrigerator for at least an hour, before removing and portioning with a hot knife.