Heat a large saucepan over a high heat, add the chorizo and fry until the natural oil has been released and the chorizo is beginning to colour. Remove with a slotted spoon, leaving behind the oil, and set aside.
Add the coriander and fennel seeds to the chorizo oil and fry for 1 minute, then add the onion and chilli and fry until the onion has softened but not coloured. Add the garlic and fry for a further minute.
Pour in the white wine and leave to bubble until just 1 tablespoon of liquid is left. Add the tomatoes and stock and bring to the boil, then return the chorizo to the pan. Tip in the clams, then cover and cook until the clams have opened. Discard any that remain closed.
Ladle into bowls, sprinkle with a few basil leaves and serve with crusty bread, if liked.