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Cook the potatoes in plenty of boiling salted water until tender, about 15 mins. Drain and allow to cool for 10 mins. Make the dressing by mixing the mayo with most of the chives.
Scatter half the watercress across a big platter or plate, chop the rest roughly and mix with half of the mackerel, the potatoes and mayo. Toss so that everything is covered in the dressing.
Pile the mackerel-potato mix on top of the watercress, then top with the rest of the mackerel and chives.