Heat butter, condensed milk, syrup and sugar and sugar in a heavy based saucepan. Stirring constantly, bring to the boil. Boil steadily for 7-8minutes until thickened, stirring from time to time to stop mixture burning.
Remove from heat and stir in the crushed biscuits. Pour the mixture into a greased shallow baking tray. Press on the marshmallows. spacing them apart evenly whilst the fudge is still warm.
Melt the chocolate and drizzle over the marshmallow and fudge mixture. Chill until set, then cut into squares.
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