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Put the noodles in a large heatproof bowl, pour boiling water over them and leave for 4 minutes, then drain and refresh under cold running water.
Put the lime juice, cayenne, sugar and fish sauce in a bowl and mix well. Have all the other ingredients ready by the cooker.
Heat the oil and fry the prawns until warmed through. Add the spring onions and noodles and toss around. Tip in the lime juice mixture, then stir in the beansprouts and half the peanuts and coriander. Cook for 1 minute until everything is heated through.
Pile into a large dish, scatter with the rest of the peanuts and coriander, and serve with lime wedges and sweet chilli sauce.