1) Bring a pan of salted water to the boil and cook the potatoes for 10 – 12 minutes until tender. 2) Drain in a colander and cool under the cold tap. 3) Mix the mustard, vinegar and sugar with the mayonnaise. Season to taste. 4) When the potatoes are cold mix with the mayonnaise, radishes and poppy seeds. 5) Top with the cress to serve.