Put the pork fillet on a board and, using a sharp knife, carefully trim off as much excess fat and sinew as possible. Cut the pork in half lengthways and then cut into thin slices. Put the pork in a bowl and toss with the five-spice powder, salt and plenty of ground black pepper. Set aside.
Half-fill a medium pan with water and bring to the boil. Add the egg noodles and swish around in the hot water with a long wooden spoon to separate the strands. Return to the boil and cook for 3-4 minutes, or according to the packet instructions until tender. Drain and rinse in a sieve under running water until cold; toss with one tablespoon of the sunflower oil and set aside.
Mix the sugar and cornflour together in a bowl and gradually stir in the soy sauce, mirin (or sherry) and 100ml/3½fl oz water. Put to one side.
Heat one tablespoon of the oil in a large non-stick frying pan or wok. Stir-fry the pork over a high heat for two minutes or until nicely browned. Tip onto a plate and return the pan to the heat.
Add an extra tablespoon of the oil and stir-fry the pepper and carrot for two minutes. Add the ginger, garlic, spring onions, peas and water chestnuts and stir-fry together for two minutes more until softened but not soggy. Tip onto a plate.
Pour the remaining oil into the pan and add the noodles. Stir-fry the noodles for 2-3 minutes, or until beginning to become crisp and golden-brown. Return the meat and vegetables and stir-fry together, tossing all the ingredients with two long handled spoons for 1-2 minutes, or until evenly mixed and piping hot.
Stir the soy mixture to combine the ingredients again and pour into the pan. Continue tossing everything together for 1-2 minutes, or until hot and glossy. Serve immediately, topped with more fried noodles or prawn crackers.