Combine the fish sauce, mirin, rice vinegar and lime juice in a medium-sized mixing bowl. Finely chop the chilli and add to the bowl. Put the prawns into the marinade and leave for an hour or so.
Peel the cucumber, slice in half lengthways and remove the core with a teaspoon. Cut the cucumber into short lengths and then into large matchstick-sized pieces, then put them in a bowl with the mint leaves and coriander.
Put the noodles in a heatproof bowl, pour a kettle of boiling water over them and set aside while you cook the prawns.
Warm the groundnut oil in a shallow pan. When it starts to sizzle, add the prawns and let them cook for 2 minutes. Remove the prawns, then pour in the marinade and leave it to bubble for a couple of minutes until it has reduced by about half.
Drain the noodles and add to the cucumber and herbs. Add the prawns, then the marinade and toss everything together.