Melt the butter, sugar and syrup in a saucepan over a low heat. Stir in the oats until completely coated. Spoon into a greased shallow round or square baking tin and spread until about 1 cm ( 1/2 inch) thick.
Bake at 180 °C / 350 °F / Gas 4 for about 25 minutes until golden. Leave to cool in the tin, then cut into squares, wedges or bars.