If any mussels are open, tap them lightly on a hard surface. If they don't close, discard them. Do the same with any cockles that remain open when tapped.
Add the cockles and the mussels to a large lidded saucepan, add the cider, put on the lid and steam over a medium-high heat until the seafood opens – discard any that don’t open.
Remove the mussels and cockles from the pan. Drain the liquid and set the cider stock to one side. Put the pan back on the heat and cook the shallot and celery until soft.
Pour the cider stock back onto the vegetables and cook until the volume of liquid has reduced by half. Stir in the clotted cream.
Add the cockles and mussels back to the pan, and bring them back up to heat. Finish with lemon juice, chervil, black pepper and salt.
Serve the cockles and mussels in shallow bowls with a wholesome chunky bread like a poppy and linseed bloomer.