Stir together the ketchup, soy sauce, sugar, cornflour and measurement water until smooth, then set aside. Heat a large wok or frying pan. Add the oil and swirl around the pan, then add the red pepper and cook for 2 minutes.
Stir in the garlic, ginger and chilli and stir-fry for 1 minute, then add the prawns and the ketchup mixture and cook for 3 minutes until thickened.
Add the noodles to the pan and cook until heated through and coated with the sauce. Squeeze over the lime juice, scatter with the spring onions and serve immediately.