Preheat your oven to 180℃ (160℃ for fan ovens). Line an 8x8 tin with baking paper.
Melt the butter, sugar and golden syrup over low heat on the stove. Once the sugar has dissolved, remove the pan from the heat and leave to cool for a minute.
Add the oats to a large bowl with the wet mixture. Mix until well combined.
Press the mixture firmly into your tin and bake for around 18-20 minutes. It will look a bit wobbly in the middle at first but will firm up as it cools.
Leave it to cool for 5 minutes then lightly score out your slices with a knife, without cutting all the way down. This will make it easier to cut clean slices later.
Let the flapjack cool at room temperature, then cut it into slices and enjoy!