Put the prawns in a bowl. Put the chilli, crushed garlic, coriander stalks and caster sugar in a spice grinder or small food processor and whizz together. Add half of the lime juice and the fish sauce, then pour this over the prawns. Allow to infuse for 30 minutes. When you are ready to start cooking, heat 1 tablespoon oil in a wok, add the ginger and spring onions and fry for 1 minute. Add the red pepper and fry for 1 minutes or until the pepper starts to soften. Add the water chestnuts and bean sprouts and toss together until the bean sprouts start to wilt. Add the soy sauce and a really good grind of black pepper, followed by the cooked noodles and sesame oil. Mix together well and tip into a serving dish. Heat the remaining oil in the wok and add the prawns, lifting them out of their juices. Toss in the hot pan for 1-2 minutes until they turn pink, add the marinade and swirl the wok quickly, then tip onto the vegetables. Sprinkle over the coriander leaves and squeeze over a little lime juice. Serve with cooked noodles immediately.