Lightly crush the digestive biscuits and chop the marshmallows in half or quarters.
In a large bowl place the broken digestive biscuits, chopped marshmallows, peanuts and M&Ms.
Melt the butter, golden syrup, dark chocolate and peanut butter together and pour through the the rest of the ingredients. Mix quickly and press into the prepared tin in an even layer.
Top with the extra M&Ms and peanuts and place in the fridge for one or two hours until set. Cut with a sharp knife into pieces and serve.