Place the chilli (reserving a little to garnish), garlic and oil in a small bowl, and cook in the microwave on full power for 1 minute. Alternatively, warm over a gentle heat in a small saucepan for about 5 minutes. Set aside.
Wash and scrub the cockles and mussels carefully, scraping off the beards from the mussels. Rinse in plenty of clean water, and discard any mussels that are open or do not close when tapped with a knife.
Bring the wine, onion and parsley stalks to the boil in a large saucepan over a high heat. Add the cockles and mussels, cover and boil vigorously for 3-4 minutes, or until all the cockles and mussels have opened. Discard any mussels that remain closed. Drain into a colander with a bowl underneath to catch all that precious cooking liquid. Reserve the broth and, as soon as they are cool enough, remove about two-thirds of the cockles and mussels from their shells.
Place both the shelled cockles and mussels and the ones in their shells in a saucepan and pour the reserved broth over them, leaving behind the last few spoonfuls of liquid, as it will be quite gritty.
Cook the spaghettini in plenty of boiling, salted water according to the packet instructions.
Meanwhile, heat the cockles and mussels in the broth, adding the chilli and garlic oil to the pan. Add the herbs, reserving some of the chopped parsley to garnish. Do not allow to boil or continue to cook as the cockles and mussels will toughen.
Drain the spaghettini, toss the pasta with the cockles and mussels mixture, sprinkle over the reserved chilli and parsley, and serve at once in warm bowls.