Add golden syrup or substitute, spices and both sugars. Bring to a simmer and cook for a few minutes to infuse the spices. Remove from heat and stir in the vanilla extract.
In a large mixing bowl combine the oats, flour and salt.
Add the butter/sugar mixture to the dry mixture. Stir well and spread into a parchment-lined Pyrex baking dish or similar approximately 10 x 15" (25 x 38cm).
Bake in the oven on a middle shelf for approximately 35-40 minutes or until golden. The original recipe indicated a baking time of 20-25 minutes but that was not long enough for the bars to brown.
Remove baking dish from oven and set aside to cool 10-15 or until firm enough to slice. Cut into squares then carefully lift the parchment paper out of the pan. Place the bars on a wire rack to cool completely.