Solid, off white cockle around 1.5cm length & width. Radiating ribs / ridges and some concentric ridges / growth lines.
No interactions present.
My Collins says 24-28 ribs :)
The books I've looked at say Parvicardium has up to 22 broad ribs; Cerastoderma has around 24 (or if you look in 60 year old books 24-28). The problem of course is the extent of variability.
A better way to separate the two species is to look at the shell teeth. Cerastoderma has two posterior lateral teeth, Parvicardium has one.
Lat/Lng: 53.3037, -4.1588
OS grid ref: SH562807