The flapjacks should last 5-7 days if kept in an airtight container.
Preheat the oven to gas mark 2 and grease a 20cm baking tin with butter, line with greaseproof baking paper.
Chop your white chocolate into small to medium sized chunks if you have not done so already. Add the butter, sugar, syrup and cinnamon to a large saucepan and melt together over a low heat until the mix is liquid and lump free.
Next remove the mixture from the heat and stir in the porridge oats. Stir in the chopped white chocolate and the dried cranberries and tip the mix into the baking tin. Using a large metal spoon, press the mixture down making sure it is spread evenly around the baking tin.
Bake in the centre of the oven for 40 minutes.
Remove from the oven, leave to cool and firm for at least an hour and then cut into 12 slices and serve.
You can use your preferred baking tin for this, it doesn't have to be 20cm, a smaller tin will give thicker flapjacks while a bigger one will make them thinner. The flapjack will seem loose when first taken out of the oven but will firm up on cooling.