Cook the prawns in salted water for about 5 minutes, then remove them, allow to cool and cut in half lengthways. Pour boiling water over the glass noodles, leave to stand for 2 minutes, pour into a sieve, rinse with cold water and cut shorter.
Mix the chilli, chopped ginger, lime juice and lime zest, sugar and oil together and season the sauce with salt. Chop the peanuts and mix in with the herbs.
Peel the grapefruits and cut out the fruit fillets between the separating skins, collecting the juice.
Mix the glass noodles, prawns, diced peppers, grapefruit fillets and the collected juice with the sauce.