Line a 15cm x 24cm | 6in x 9½ baking tin with baking paper.
Blitz half the biscuits in a food processor until you have fine crumbs. Tip them into a large bowl. Break the rest of the biscuits into small pieces and add to the bowl.
Put the butter, golden syrup and the chocolate in a small pan. Allow it to melt slowly over low heat.
Pour the melted chocolate over the biscuits and stir until the biscuits are coated.
Tip into the prepared tin and press down with the back of a spoon until you have an even layer. Place in the fridge to set for 30 minutes.
Make the caramel: put the double cream into a measuring jug and heat for 20 seconds in the microwave until warm.
Put the sugar, honey and water in a small deep saucepan. Bring slowly to the bowl over medium-low heat, swirling the pan until the sugar melts.
When the sugar turns a golden amber, remove from the heat and add the warm cream carefully. The mixture will bubble alarmingly and then settle.
Stir in the butter and salt and bring the mixture back to the boil for a couple of minutes. Strain the caramel into a bowl and leave to cool until just warm.
Pour the caramel evenly over the biscuit base. Place back in the fridge to set for about an hour.
When the caramel has set, it's time to make the chocolate topping. Bring a small pot of water to the boil then reduce to a simmer. Place the chopped chocolate and butter in a bowl and set over the pot. Allow them to gently melt then set aside.
Melt the milk chocolate and double cream using the same double boiler method and set aside.
Pour the dark chocolate over the caramel layer and spread evenly.
Dot blobs of the milk chocolate on dark chocolate topping then use a chopstick to mix the two layers to create a feathered effect. Return to the fridge until completely set.
Warm a large knife until hot water and wipe dry. Cut the chocolate tiffin into bars or squares, wiping the knife clean between cuts.
Keep the bars in the fridge or consume immediately.