Line a 20x30cm/8x12in baking tin with baking paper.
Place the dark and milk chocolate pieces in a large heatproof bowl and add the golden syrup and butter. Set the bowl over a pan of barely simmering water but do not allow the bottom of the bowl to touch the water. Stir from time to time until the butter and chocolate are melted and the mixture is smooth. Remove from the heat and leave to cool for 5 minutes – if this is too hot the marshmallows will melt.
Cut the chocolate mini eggs in half (leaving a few whole) and set aside one third of the eggs, including the whole ones for decorating. Gently fold in the chocolate eggs, biscuits, marshmallows, nuts and raisins into the chocolate mixture and mix to combine. Spoon into the prepared tin and spread level using the back of the spoon. Scatter with the reserved whole and halved mini eggs.
Refrigerating for at least 1 hour, or until completely firm. Cut into bars to serve.