Put all the sweet and sour ingredients, except the corn flour and tomato ketchup, in a saucepan, bring to the boil and leave to simmer for 30 minutes.
Then stir in the corn flour mixture and cook until thick and glossy.
Drop the noodles in a large pan of lightly salted boiling water, bring back to the boil and cook for 4 minutes.
Drain and cool under cold water.
Divide the pork into four portions and warm on a griddle pan for 4 minutes on each side.
Then heat half the mixed oils in a large frying pan or wok, add the cold, drained noodles and stir-fry until lightly coloured.
Transfer to a plate and keep warm.
Add the remaining oil to the wok and stir fry the vegetables for 2 minutes, starting with the mushrooms and sweetcorn, then adding the pepper, garlic and ginger, then the spring onions, mangetout and finally the carrots.
Add the noodles, sweet and sour sauce, coriander, fish sauce and soy sauce to taste and mix well. Serve with the warm pork belly.