Hugh Fearnley Whitigstals Three good things. An excellent book.
Clams were replaced by cockles, although three clams made it in. All of which were hand gathered by myself. Good tomatoes are a must! If you can only get naff ones grown hydroponically in Holland or the huge place in Kent, try another recipe. A good white wine, The UK is doing some good white wine, however I used Noilly Pratt. Good butter, I used a Normandy butter.
The purging took 48hrs, with 5 changes of water. The overnight purge is a myth. I tried it, the result was grit city. Even 36hrs ended up gritty. 48hrs was almost perfect. I think temperature also plays a part. To cold, slows things down. At night they moved in to the kitchen, not the minus degrees, of the garage.
I used course polenta and oatmeal (that’s not rolled oats). This either feeds or irritates the cockles or both perhaps.
Oxygen is the thing most needed. So use a fast running tap, when filing the cockle bath. I use a washing up bowl, with a roasting rack in it. It works! Change the water when you get up in the morning or every 7, 8, 9, 10 hrs. Even I have to work and sleep.
This web site has the best info aimed at commercial purging of shellfish.
As with most experiences with food, the same rules apply to small time foragers like myself.
I hope you try this.