PUT THE NOODLES into a heatproof bowl and cover with boiling water. Cover and leave to one side for 2–3 minutes to soften. Once soft, drain in a colander and rinse with cold water. Drain again and squeeze out any excess water with your hands so that they are really dry. Leave to one side.
MAKE THE DRESSING while the noodles soften. Remove the tough outer leaves from the lemongrass and cut off the ends of the stalks. Starting at the fatter end, roughly slice each lemongrass stalk into rings. You should see a purple band in the rings. Stop slicing when there are no more purple bands, then discard the rest of the lemongrass, as it will be too tough to eat.
GIVE THE LEMONGRASS SLICES a quick blast in a mini food processor until they are very finely chopped, then tip them into a large mixing bowl. Squeeze the juice from the limes into the bowl with the lemongrass and add the chilli powder, sugar and oil. Whisk everything together to get the flavours going.
USING A SHARP KNIFE, cut the top and bottom off the grapefruit, then stand it up on your chopping board. Carefully slice the skin off in sections, cutting from top to bottom. Remove any remaining pith, then cut out the juicy pieces of grapefruit from the membrane and put them into the bowl with the dressing. Squeeze in the juice from the membrane. Trim and finely slice the spring onions, then add them to the bowl. Chuck in the pea shoots, crayfish, basil, mint and cooked noodles. Toss everything together well. Serve with the pumpkin seeds scattered over the top.