Place the stock and water into a large pan over a high heat and bring to the boil.
Add the lemongrass, coriander roots, galangal, tomatoes, lime leaves, lime juice, tamarind water and red chillies. Return to the boil, then reduce the heat and simmer for 15-20 minutes.
Taste the stock and add as much fish sauce and palm sugar as necessary to keep the salty-sour-hot-sweet taste, then remove from the heat.
Transfer 220ml/8fl oz of the stock into a separate pan over a high heat and bring to the boil. Add the chicken and cook for 1-2 minutes, then add the prawns and cook for a further 1-2 minutes, or until completely cooked through.
To serve, pour the soup into two bowls and top up with the hot stock. Garnish with Thai basil, coriander and a wedge of lime.