Preheat your oven to 180℃ (or 160℃ for fan ovens) and line an 8x8 tin with baking paper.
Add the butter, sugar and golden syrup to a saucepan. Place pan over low heat on the stove and mix gently until the butter has melted and the sugar has dissolved.
Pour the mixture into a large bowl along with the oats and cinnamon. Mix well until the oats are evenly coated.
Stir in the raisins.
Press the mixture firmly into your tin and bake for around 20 minutes. The flapjacks will be golden at the edges and a little wobbly in the middle. Don't overbake them or they'll be hard once they cool.
Let the flapjack cool in the tin for around 5 minutes then use a knife to gently score lines where you plan to cut it, to prevent crumbling later. Leave to cool completely.
Remove the cooled flapjack from the tin, cut into slices using your score marks as a guide, and enjoy!