Mix the chicken with half the lemon juice and the sweet chilli sauce in a large bowl and season to taste with salt and pepper.
Lay a chicken breast between 2 sheets of clingfilm and lightly pound with a mallet to flatten. Repeat with the remaining chicken breasts.
Arrange the chicken on a grill rack in a single layer. Cook under a preheated grill for 4–5 minutes on each side or until cooked through. Finish on the skin side so that it is crisp.
Meanwhile, put the noodles in a heatproof bowl, pour over boiling water to cover and leave for 10 minutes until just tender, then drain. Add the remaining lemon juice, herbs and cucumber to the noodles and toss well to mix. Season to taste with salt and pepper.
Top the noodles with the cooked chicken and serve immediately, garnished with the chopped red chilli.