Boil the potatoes in plenty of boiling water until just tender. Drain and leave in the colander until they are cool enough to handle. Mix the mayonnaise, Chinese vinegar, sesame oil, garlic, wasabi and lemon juice.
Break the potatoes gently with your fingers into bite-sized pieces. Stir the wasabi mayo mix gently-gently through the still-warm potatoes and arrange on the platter, layering generously with the finely sliced spring onions, radishes and herbs so that all the potatoes get a bit of all this colour and seasoning.
Once all the potatoes have been layered, garnish with the remaining chopped herbs, ribboned cucumber, radish and watercress.