Lightly grease a Swiss roll tin and set aside.
Melt the margarine and syrup (plus sultanas if using so that they swell) in a small saucepan over a low heat. Add cocoa and stir. Stir in the crushed biscuits (and nuts if using).
Press the mixture into the Swiss roll tin and put in the fridge for about an hour. After an hour, melt chocolate in a basin over a saucepan of hot water. Spread the melted chocolate evenly over the biscuit mixture.
Place the tin back in the fridge until the tiffen is set then cut into required size pieces. Keep in fridge.
Remove from fridge about 10-15 minutes before eating.
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