Blend the garlic, shallots, chilli paste and shrimp paste together in a blender or food processor.
Place a wok over a high heat and fry the blended paste until the oils separate.
Add the lemongrass, tamarind, lime leaves, 100ml/3½fl oz water and sugar and cook.
Discard any clams with broken shells and any that refuse to close when tapped.
Season the lemongrass mixture with salt and pepper.
Stir in the clams and prawns and cook for a few minutes before adding the squid.
Discard any clams that remain closed and check that prawns and squid are cooked through.
Stir through the basil and serve.
