Heat the oil in a large saucepan and add the peppercorns, lemongrass, cinnamon, star anise and root ginger and cook, stirring constantly for 1-2 minutes to aromatise the oil. Stir in the concentrated stock, 400ml water and chicken breast. Bring to the boil and simmer very gently for 25 minutes. Lift the chicken out of the stock using a slotted spoon and remove and discard any spices sticking to it. Strain the stock (discarding the spices) and return to a clean saucepan along with the chicken. Slice into the chicken at the thickest point to ensure that it is cooked through and showing no pink inside. Return to the heat and add the Thai fish sauce, lime juice, noodles and stir fry vegetables and simmer for 3 minutes. Stir in the spring onions and divide between 4 serving bowls. Scatter with the red chilli and coriander leaves and serve with an extra wedge of lime to squeeze over if liked. Tip: This soup works well with fish, beef or pork. Simply replace the chicken with 250g sirloin steak or pork fillet cut into thin strips and stir fry for 2-3 minutes over a high heat and add to the soup with the noodles. Or add a 200g raw peeled jumbo prawns (thawed if frozen). This recipe is not suitable for freezing.