Tip the noodles into a bowl then cook in the microwave according to the pack instructions.
While cooking make a dressing. Crush the tomatoes with a wooden spoon and mix with the fish sauce, lime zest and juice, sugar, chilli, and 4 tbsp grapefruit juice. Toss into the hot noodles and leave to cool completely or until they are warm.
Add the remaining ingredients, mix very thoroughly and serve.
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