Rinse the potatoes in cold water, then boil them for 20 minutes or until tender. Drain and allow to cool briefly. Cut into quarters.
Place potatoes in a large bowl with the flaked mackerel, crème fraîche, horseradish cream and lemon juice. Toss gently to fully coat the potatoes and mackerel with dressing.
Add torn pieces of watercress and mix again just before serving.
Serving Suggestions: Great on its own or with extra watercress and a drizzle of olive oil.