Preheat your oven to 180℃ and line an 8x8 tin with baking paper.
Melt the butter, condensed milk, sugar and syrup together over low heat on the stove. When the sugar has dissolved, remove the pan from the heat and leave to cool for a minute or two.
Pour the oats into a large bowl along with the wet mixture. Mix well until all of the oats are evenly coated.
Transfer the oaty mixture to your lined tin and press it firmly into place.
Bake for around 18-20 minutes, until the flapjack is golden around the edges.
Let it cool for around 5 minutes then use a knife to lightly score the flapjack where you'll slice it later.
Leave to cool fully, for 3-4 hours, then cut it into slices and enjoy.