Cook the miniature new potatoes in a large saucepan of salted, boiling water until tender; usually 20 minutes.
Meanwhile, cook the eggs in a saucepan of salted water for 12 minutes. Drain and run under cold water for 1 minute. Drain the potatoes when they are tender and run under cold water for 2-3 minutes until cool enough to handle. Peel the eggs and cut in half.
Combine the potatoes with the peas in a large mixing bowl. Add the olive oil and some seasoning and toss well. Blanch the sprouts for at least 15 seconds in boiling water and allow to cool. Spoon into plastic containers and garnish with the egg and the watercress sprigs. Serve cold.
If serving vulnerable groups, elderly people, toddlers, pregnant women and people who are unwell, cook eggs until the whites and yolks are solid.