Line the base of a 20cm (8in) square cake tin with nonstick baking paper. Put the chocolate, butter and golden syrup in a bowl over a pan of simmering water, stirring occasionally until it's melted, smooth and shiny. Leave to cool to room temperature.
Put the biscuits, cherries and two-thirds of the marshmallows in a large bowl and mix well, then pour over the cooled chocolate mixture. Stir until all the ingredients are evenly coated.
Tip the mixture into the prepared tin and even it out with the back of a spoon. Scatter over the remaining marshmallows. Chill for at least 3 hours or overnight until firm.
Run a knife around the edge of the tin, turn out onto a board and remove the baking paper. Turn it the right way up and cut into 36 squares.
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