Combine all the ingredients for the beef marinade well and baste liberally on the steaks. Leave covered for at least an hour.
Make your sauce by combining all the ingredients, whisking well to dissolve the cornflour.
When you're ready to cook, get your wok on screaming hot. Shake excess marinade off the beef and fry the steaks for 90 seconds on each side, until they have coloured well but still quite squishy to the touch. Leave on a board to rest on one side while you carry on with the rest.
Keep the wok hot and add a tablespoon of oil. Stir fry the vegetables until starting to soften. Keep stirring to avoid them catching, and then add the sauce (at this point you don't have to stir so diligently).
Slice the steak thinly. When the sauce in the wok is bubbling and thick, tip the steak and the cold noodles into the pan. Stir fry for another minute to warm up the beef and noodles, and serve immediately.