Soak the noodles in a bowl of warm water until soft, then drain and put back in the bowl. In a hot wok fry the beef and five-spice in the olive oil until brown and crisp, then add the garlic, ginger, prawns and sugar and stir-fry for another 4 minutes. Remove from the heat and stir the wok mixture into the noodles. Add the spring onions, lime juice, fish sauce, chillies, coriander, mint and peanuts to the bowl. Toss well and correct the seasoning — it wants to be quite zingy with the lime juice. Sprinkle with some extra herb leaves if you like and serve cold.