Put the orange juice and curry paste into a bowl and stir together. Add the couscous, sultanas and a little pepper, then pour in the boiling water and fork together. Leave to stand for 5 minutes.
Meanwhile, peel the skin off the mackerel fillets and break the flesh into large flakes, discarding any bones.
Add the mackerel, onion, red pepper and tomatoes to the couscous and mix together lightly. Sprinkle roughly chopped coriander over the top, spoon on to plates and serve immediately.