Preheat the oven to moderate 180°C Line a shallow 28 x 18cm tin with aluminium foil; brush with oil or melted butter.
Sift flour and sugar into a large mixing bowl. Add oats and coconut; make a well in the centre.
Combine butter and golden syrup in a small pan. Stir over low heat until butter has melted and mixture is smooth; remove from heat. Dissolve soda in water; add immediately to butter mixture and stir until foamy. Add to dry ingredients and stir until well combined.
Spread mixture into prepared tin, pressing firmly with the back of a spoon to smooth surface. Bake for 25-30 minutes, until golden brown. Cool in tin then, using foil, lift out and cut into squares.