Preheat the oven 350°F (180°C), Gas mark 4. Grease a shallow 7in (18cm) round or square cake tin. Melt the butter with the sugar and syrup in a pan over a low heat, then pour it on to the rolled oats in a large bowl. Mix well, then scrape the mixture into the prepared tin and press down well.
Bake the flapjacks for about 20 minutes until golden brown. Cool slightly in the tin, then mark into slices or fingers with a sharp knife and loosen round the edges. When firm remove from the tin, and cool on a wire rack, then break into slices or fingers.
To serve, arrange the flapjacks on a large plate and allow guests to help themselves.