Bring the water to the boil. Add salt, pour in the rice and stir. Cover and lower the heat. Cook gently for around 15 minutes. The rice should be cooked but still retain some bite. Pour it into a sieve and stop the cooking by rinsing under cold running water, drain thoroughly and leave to cool.
Rinse the cherry tomatoes and carefully wipe them dry. Drain the sun-dried tomatoes and cut them into fine strips. Thinly slice the red onion. Rinse and pit the olives. Wash the basil leaves and shred finely.
Make the vinaigrette: place the vinegar in a bowl and add the mustard, olive oil and salt. Whisk rapidly.
Arrange the rice, mangetout, cherry tomatoes and sun-dried tomato strips in a salad bowl. Add the onion, olives and basil; drizzle with the vinaigrette and garnish with a good grinding of pepper. Serve well chilled.