Bring 1L water to the boil in a large saucepan over high heat. Stir in stock cube to dissolve and add rice; cook according to rice packet directions. Rinse with cold water and drain; cover and refrigerate for at least 30 minutes to chill.
Meanwhile, prepare the dressing: In a small bowl whisk together the oil, vinegar, lemon juice, salt and pepper. Cover and set aside.
To make salad: In a large bowl combine the courgette, avocado, tomatoes and chilled rice. Pour dressing over salad and toss; finally, add prawns and parsley.
Cover and put in refrigerator for at least 30 minutes; serve chilled.