Heat the butter, chocolate (in pieces) and golden syrup until liquid, making sure to mix about every 30 seconds; I use the microwave.

While heating crush the biscuits into about 3cm pieces.

Once the chocolate mixture is ready, pour 125ml into a bowl and mix the biscuits and marshmallows into the rest.

Tip and spread the biscuit mixture into a 24cm tin.

Pour the remaining 125ml on top and spread.

Then leave out until cool and then put into the fridge until totally set; usually about 3 hours is fine but overnight is best.