1. Cook noodles according to packet instructions.
2. Drain, then refresh under cold running water. Set aside.
3. In a small saucepan, combine the sugar and water, stirring (over low heat) until sugar dissolves.
4. Turn heat to high and bring to a boil.
5. Reduce heat and simmer until sauce thickens slightly (about 2 minutes).
6. Remove from heat, then stir in vinegar and chili.
7. Pour sauce into 4 small sauce bowls. Set aside.
8. Heat half the oil in a wok.
9. Add chicken (in small batches) and stir-fry until browned.
10. Add remaining oil to wok.
11. Return the chicken to the wok along with the lemon grass and garlic and stir-fry until fragrant (about 1 minute).
12. Add fish sauce and stir-fry to coat chicken. Remove from heat.
13. Divide noodles between 4 wide shallow serving bowls.
14. Arrange salad ingredients around the bowl and placing lemon grass chicken in centre.
15. Sprinkle with chopped peanuts and grated radish.
16. Serve with chili dipping sauce alongside.