Soak the noodles in boiling water for 6-8mins, or according to pack instructions. Meanwhile, mix together the lime zest, lime juice, soy sauce and sweet chilli sauce in a large salad bowl. Add the shallots and prawns, stir gently, then leave to marinate for about 10mins.
Drain the noodles and add them to the bowl. Toss everything together, then serve.
Try using a papaya instead of mango - it's a popular Thai ingredient. Peel it, halve it, then scoop out the black seeds before slicing it.