Grease your baking tin and then line it with greaseproof (baking) paper, crossing over so that both sides and the base of the tin are covered.
Add your butter and syrup to your small saucepan and melt slowly over a low heat.
Mix in your dark and milk chocolate until totally melted.
Leave to cool.
Add your marshmallows, malted milk biscuit pieces and raisins and mix in. You may find it easier to mix these separately first and then add them all together to ensure they are evenly mixed in.
Add your Maltesers and mix thoroughly together.
Place in your prepared tin and spread out evenly.
Refrigerate and for 3-4 hours until totally set.