Cut the chicken breasts into thin slices, add to the marinade, cover and refrigerate for at least one hour.
Heat wok until very hot, add the sesame seeds and dry-fry them until just golden, then remove them from the wok. Set aside.
In a separate saucepan add the noodles and cook according to packet instructions, then drain.
Add sesame oil to the Wok and heat gently.
Using a slotted spoon remove the chicken from the marinade, add the chicken to the wok and stir-fry for about 5 minutes.
Add the marinade to the Wok along with the leeks and stir-fry all the ingredients until the leeks are soft for about 2 – 3 minutes.
Check the chicken is cooked, add the sesame seeds and the drained, cooked noodles, stir-fry together well for about 30 seconds, season with salt and pepper.