In a large saucepan, heat the golden syrup and sugar together, bring to a boil then simmer on low heat for 10-15 minutes. Cooking time will vary but test it by dropping a little syrup into water-it should become brittle when ready. Watch it doesn't burn.
Remove pot from heat and add the bicarb soda. Quickly mix it in because the mixture will foam up instantly. Pour immediately into the cake tin. Leave to set then break into bite size chunks.