Melt the butter with the golden syrup, condensed milk and sugar in a large pan set over a medium-low heat, then remove from the heat and stir in the oats, raisins and flour. Stir well.
Scrape the mixture into a greased and base-lined baking tin measuring about 25 x 25 cm (10 x 10 inches), with a depth of about 3.5 cm (1 1/2 inches). Bake in a preheated oven, 180˚C (350˚F), Gas Mark 4, for 15–18 minutes until pale golden in colour.
Remove from the oven and cool in the tin for 2–3 minutes before marking out and cutting about 16 squares or bars. Allow to cool for about 5 minutes or until cool and firm enough to handle, then transfer to a wire rack.