Butter and line a 20cm square tin with non-stick baking parchment. In a large bowl, combine the biscuits and nuts, halving any larger nuts. Melt the chocolate, butter and golden syrup in a bowl set over a pan of simmering water, stirring occasionally until smooth and glossy, then pour this over the biscuit and nut mixture.
Tip the mixture into the tin, then flatten lightly – it doesn’t need to be completely smooth. Chill for at least 2 hrs or overnight before cutting into squares.