Simmer the fish bones in 1.5 litres of water, along with the smashed lemongrass, galangal and half the lime leaves for about 20 minutes, then sieve into a clean pan, discarding all the bits. Stir in half the fish sauce, half the lime juice, half the sugar and half the chillies, and leave to bubble for a couple of minutes. Add the prawns and the fish pieces and simmer gently for 2-3 minutes or so, until the fish is cooked. Give it a taste and add more flavourings until it’s blended in heat and sourness to your liking. Remove and discard the lemongrass and spent lime leaves. Stir in the remaining fresh kaffir lime leaves, remove from the heat and add the remaining lime juice. Divide between bowls and top with leaves. Again, toasted shrimp paste mashed with chilli can be added at the table.