Reduce the remaining saved cooking liquid by around half over low heat before using in your recipe.
Remarks
When preparing cockles, you will encounter two problems: sand, which is very unpleasant in the mouth (so do follow the advice to deal with this) and the saltiness of the cooking liquid. If you use this to make a sauce, do not add any salt.
Cockles are mostly shells and water: for 1 kg of cockles you will only end up with around 100 g of useful cockle meat (90% waste).