Cut the pork fillet in half lengthways, then across into very thin slices, discarding any areas of excess fat. Place in a bowl and drizzle with 2 tablespoons of the soy sauce, the five-spice powder and honey. Mix well, cover and marinate in a cool place for 30 minutes.
Mix the cornflour with the measurement water in a cup until smooth. Stir in the remaining soy sauce.
Heat 1 tablespoon of the oil in a wok or large frying pan over a high heat. Add the pak choi and spring onions and stir-fry for a couple of minutes until the greens have wilted. Lift out on to a plate. Add the remaining oil to the pan, tip in the pork and fry, turning once, for 3 minutes or until cooked through. Add the ginger, chilli and garlic and cook for a further 1 minute.
Return the vegetables to the pan with the water chestnuts, noodles and soy sauce mixture. Cook, stirring constantly, for 2–3 minutes until hot and the sauce is thickened and glossy. Serve immediately.