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Light the barbecue and allow the flames to die down until the ashes have gone white with heat. Make the drizzle by whisking 2 tbsp olive oil and all the other ingredients together in a small bowl, adjusting the ratio of honey and lime to make a sharp sweetness. Season to taste.
Score each side of the mackerel about 6 times, not quite through to the bone. Brush the fish with the remaining oil and season lightly. Barbecue the mackerel for 5-6 mins on each side until the fish is charred and the eyes have turned white. Spoon the drizzle over the fish and allow to stand for 2-3 mins before serving.