Preheat oven to 350 degrees F. Butter 8x8x2-inch good quality metal baking pan. Or lightly butter pan, line with parchment paper, then butter the parchment.
Combine first butter, brown sugar, and Lyle's Golden Syrup in heavy medium saucepan. Stir constantly over medium-low heat until butter melts, sugar dissolves, and mixture is smooth. Remove from heat.
Add oats and salt; stir until oats are coated. Transfer mixture to prepared pan and spread out in even layer. I used a wooden spoon to press gently, then used my fingers to finish pressing the edges and corners.
Bake until top is golden and bubbling (edges will be darker), about 25 minutes (but check sooner to make sure it is not burning). Cool in pan on rack 5 minutes.
Cut into 4 squares; cut each into 4 triangles (mixture will still be soft). Cool completely in pan before serving. Use your handy-dandy parchment paper to lift flapjacks out of the pan. No cursing or sharp metal objects necessary.
Notes
UPDATE: I did try the parchment and it was MUCH easier to get the flapjacks out of the pan.
Recipe by story of a kitchen at https://www.storyofakitchen.com/dessert-recipes/flapjacks-oatmeal/