Mix together the soya sauce, vinegar, plum sauce and tomato paste in a small bowl and set aside.
Heat the oil in a wok and stir-fry the spring onions and ginger over a fairly high heat for about 30 seconds. Add the duck and stir-fry over a high heat for about 2 minutes until lightly cooked.
Add the capsicums to the wok and continue to stir-fry for 4–6 minutes until the capsicums and duck are just tender and the juices have evaporated. Add the kumquats (if using) and cook for a further 1 minute.
Stir the sauce mixture, then pour it into the wok, tossing everything together to coat with the sauce and heat through. Add the pek chye and stir-fry for about 1 minute or until wilted.
Finally, add the noodles to the wok and stir-fry for 2–3 minutes until thoroughly heated. Serve immediately.