Place the potatoes in a pan of cold water. Bring to the boil then simmer for 12-15 minutes, until tender. Drain and allow to cool. Meanwhile, place the asparagus in a pan of boiling water and simmer for 1½ minutes, then drain and refresh in cold water.
Put the white wine vinegar, lemon juice, caster sugar, oil and mustard powder in a jug. Season with freshly ground black pepper, whisk together, then set aside or store in an airtight container until required.
Transfer the potatoes and asparagus to a large bowl and add the onion, cucumber, radishes, parsley and watercress. Toss to combine.
To serve, gently toss through the dressing and top with the flaked mackerel.
Cook's tip: you could always use a drained 200g tin of tuna steak in water in place of the mackerel in this dish.