Pre-heat your oven to 180°c (160°c for a fan-assisted oven or Gas Mark 4). Grease a 11x7 inches tray bake tin with some excess butter/margarine, before lining with greaseproof paper. Set aside.
In a large pan, warm the butter, golden syrup and sugar - stirring regularly - until melted together. This can also be done in the microwave, just use a microwave-proof dish and remove to stir frequently.
In a large bowl, lightly mix the oats and cinnamon. Add the oats to the melted mixture and stir together until all the oats are coated.
Transfer the mixture to your prepared tin and smooth the mixture into the corners.
Bake in your pre-heated oven for about 22-25 minutes, until the edges are lightly golden and the middle still has a "slight wobble". It may look like it has not completely baked, but don't worry! It will set as it cool downs.The edges will have risen a little, so press back down with a cold spoon, if desired.
Leave to cool completely in the tin, before lifting out in the greaseproof paper. Gently peel back the edges and cut into your desired sizes. The flapjack can sometimes be a little chewy in the middle and need a little persuasion to lift without breaking - a small spatula can be used for this purpose.
Notes
For full explanations on each ingredient and possible substitutions, see the Ingredients section above.
For more tips for success in making the perfect flapjack, see the Granny's Top Tips section above.
For hacks and advice on making free-from and vegan flapjack, see the Free-from & Vegan section below.
Store your flapjack in an air-tight container, lined with additional greaseproof paper. Flapjack slices will keep for up to 1 week this way, or you can freeze it for up to 3 months.