Scrub and wash the potatoes. Cut them in half and put them in a pan of cold water. Bring to the boil then simmer for about 10 minutes until tender. Drain then transfer to a serving bowl.
Meanwhile, put the beans and peas in a bamboo steam basket and put them over the cooking potatoes (or blanch them for 2 minutes in boiling water).
Slice the radishes. Mix the dressing.
Rinse the vegetables under cold water, drain , then add to the potatoes together with the radish and watercress. Gently toss with a large spoon.
Just before serving, drizzle the dressing over the salad and season to taste.