Mix broken into pieces biscuits,pistachios, glace cherries and maltesers buttons in a large bowl.
Melt chocolate, butter and golden syrup in a double boiler over low heat until just melted, stir to combine. Pour the chocolate mixture into the bowl with biscuits and nuts and gently stir to combine.
Turn the mixture out into a parchment lined 9X13 inch pan and level. Put in the fridge for 2-3 hours to set.
Heat the double/heavy cream in a saucepan until nearly boiling, then pour it over chopped dark chocolate, let it sit for a couple of minutes, then stir to combine. Pour the ganache over the chilled fridge cake and spread evenly with an offset spatula. Let it cool, then chill in a fridge for at least an hour.
Take the fridge cake out of the pan and peel the parchment paper off. Cut into thirds crosswise, then take each individual strip and cut into triangles to resemble Christmas trees.
To decorate, melt the chopped white chocolate until runny. The easiest way is to melt it is in the microwave in 30 second spurts to prevent it from burning. Stir after 30 seconds to see if it's fully melted. Pour in a small plastic bag and snip a corner off, then drizzle over the Christmas trees. You may also use sprinkles before the chocolate is set. Keep in an airtight container in a fridge or freezer.