Preheat the oven to gas 5, 190ºC, fan 170ºC. In a blender, blitz the ginger, garlic, inner leaves of the lemongrass (reserve the stripped outer leaves), chilli, lime zest and juice (reserve the lime skins) and 1 tablespoon of oil until just puréed. Season with a little salt.
Lift the skin from the neck end of the chicken and push your hand in between the skin and the meat, separating them as far over the chicken and down into the legs as you can. Push the paste under the skin and pat to distribute evenly. Put the chicken in a roasting tin, rub with the teaspoon of oil and season with a little salt and pepper. Put the lime skins and outer leaves of the lemongrass in the cavity. Put in the oven and cook for 1 hour 20 minutes, until cooked through or the juices from the leg run clear when pierced with a skewer.
Just before the chicken is ready, mix the lime juice, fish sauce and sugar in a small pan and warm gently to melt the sugar. In a bowl, add the rest of the ingredients for the noodles and toss together with the warm sauce. Serve with the chicken, spooning over some of the chicken juices.