Pre-heat your oven to 180°c (160°c for a fan-assisted oven or Gas Mark 4) and line a 11x7 inches tray bake tin with tin foil and grease generously with butter. Set aside.
In a large pan, warm the butter, golden syrup and sugar - stirring regularly - until melted together. This can be done in the microwave, just use a microwave-proof dish and remove to stir frequently.
In a large bowl, lightly mix the oats and cinnamon before adding the melted mixture and stirring until all the oats are covered.
Transfer the mixture to your pre-lined/greased tray and smooth the mixture into the corners.
Bake in your pre-heated oven for about 20 minutes, until the edges are lightly golden and the middle still has a "slight wobble". The edges will have risen a little, so press back down with a cold spoon if desired.
Leave to cool a little in the tray before removing, carefully peeling off the tin foil and cutting into your desired sized slices.