Mix together the cumin, coriander, cayenne and turmeric
with the lemon juice and season really well. Make diagonal
slashes in the skin of the mackerel and rub the spice mix all
over. Leave for 20 minutes to marinate.
To make the salad, boil the potatoes for about 20 minutes
until tender. Toss with the other ingredients while warm.
Grill the mackerel skin-side up for about 7 minutes until
cooked through. Serve with the potato salad.