Preheat the oven to 180C. Grease and line the bottom and sides of an 8" / 20cm x 20cm square baking tin with greaseproof / parchment paper.
In a large saucepan over a low heat, place the unsalted butter, sugar, and golden syrup. Stir and melt these ingredients together. Remove from the heat.
Stir in the oats and cranberries mix well together. Allow to cool.
Meanwhile roughly chop up 80g of the white chocolate and when the oats are cooled stir in the white chocolate pieces.
Scrape the contents of the saucepan into the baking tin and press down all over with the back of a spoon ensuring an even covering.
Bake for 15-20 minutes until golden brown.
Once cooked remove from oven and leave in the tin placed on a wire rack to cool.
Once cooled melt the remaining white chocolate and spread over the top of the flapjack.
Allow the chocolate to set then carefully remove the oat slab from the tin, place on a wooden board and cut into 9 squares.