Heat oven to 150C/130C fan/gas 2. Place the sugar, butter and golden syrup together in a saucepan and gently heat until the butter has melted, stirring occasionally. Take the saucepan off the heat and stir in the oats, mixing thoroughly. Pour the mixture into a lined and lightly greased 22cm square tin, pressing it out evenly using the back of a wooden spoon. Bake in the centre of the oven for 40-45 mins.
Allow the mixture to cool in the tin for 10 mins (or for best results, leave to cool completely overnight), then evenly spread over the caramel. There can be discrepancies between the thickness of caramel, and the more you stir the runnier it gets. If it looks too thin pop it into a pan and boil for a few minutes, stirring. Then leave to cool - it should be thicker. Chill until firm.
Melt the chocolate in a heatproof bowl over a pan of barely simmering water, then stir in the oil and pour over the chilled caramel flapjack base. Let the chocolate set, then cut into squares. Setting chocolate on top of something squidgy can make it difficult to cut, this is why the oil is added.