Heat the oil in a wok over a medium-low heat. Fry the chicken 10 mins or until cooked through. Transfer to a plate; cover loosely with foil.
Reheat the wok over a high heat, add the peppers and chopped courgette; stir-fry for 5 mins. Meanwhile, bring a pan of water to the boil, crumble in the stock cube, then add the noodles. Simmer for 4-5 mins until just cooked, then drain well.
Add the garlic to the wok; stir-fry for 2 mins. In a bowl, mix the soy, sugar and the lemon zest and juice. Add to the wok, bubble for 2 mins, then add the baby marrow ribbons and noodles. Toss to coat in the sauce.