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Cerastoderma glaucum
Live small cockles (new recruits of a breeding population) have now been discovered at depths of 20 cm in Widewater, which was about a metre deep near the bridge. This is the Lagoon Cockle, Cerastoderma glaucum, although when the cockles are small (12 mm width) they do not have the shape of the full grown ones, so they initially looked to me like Common Cockles, Cerastoderma edule. Actually, whilst they are alive there does not seem to be an easy way to distinguish them as it is the interior groove differences that are diagnostic. The shell is thinner and the Lagoon Cockle is brittler and the live shell can be easily prised open with a fingernail which is much harder with the Common Cockle, even small ones. The Shore Crab, Carcinus maenas, would find these shells very easy to crack open.
BMLSS Cockles
http://www.glaucus.org.uk/Cockles.htm