Preheat your oven to 180℃ (or 160℃ for fan ovens) and line an 8x8 tin with baking paper.
Melt the butter, sugar and golden syrup together over low heat on the stove.
Pour the oats into a large bowl with the wet mixture and mix well.
Press the mixture firmly into your tin and bake for around 18-20 minutes. The flapjacks should be very slightly golden at the edges and wobbly in the middle. They'll firm up as they cool so don't worry.
Let them cool in the tin for around 5 minutes then use a knife to gently score lines where you plan to cut them. This will help to prevent crumbling later.
Leave the flapjacks to cool completely then pop them in the fridge for 2 hours to firm up. Then remove them from the tin and cut into slices using your score marks as a guide.
Video
Notes
Store in an airtight container in the fridge for up to 1 week.