Line a 20cm x 20cm brownie tin with foil. Crumble the biscuits and the walnuts into a large bowl (you want a good mixture of pieces and fine crumbs). Snip the marshmallow into halves or quarters with scissors and add those to the bowl too. Set aside.
Put the butter and golden syrup in a saucepan and heat gently until the butter has melted. Put the chopped chocolate in a heatproof bowl then pour the melted butter and syrup mixture over it and leave to sit for a couple of minutes without stirring. Now stir until the chocolate is smooth and has melted then pour the whole lot over the biscuits and marshmallows. Stir to coat all the pieces in the chocolate then pour into your prepared tin in an even layer. Don’t press it down too much, you want quite a bumpy 'rocky road' surface. Leave to set in the fridge for 1-2 hours or overnight then dust with icing sugar and chop into bars or squares to serve. Keeps in the fridge for up to a week.