I think I would just call them clams? If so, they are very common, easy to dig and lovely to eat. Similar habitat to cockles, but usually need a lower tide and to dig a bit deeper. Often see them squirt as the siphon disappears.
Yes, they're the biggest 'clams' we have, complete with siphon etc. Mya arenia apparently.
Roughly how deep Crofter? Thinking about it now I have seen people digging at low tide when I've been out collecting razor clams. I always assumed they were just making hard work for themselves (or digging lugworm for bait) but there are gaper shells all over that beach.
I am thinking about smaller clams, Venerupis pullastra or Venerupis senegalensis (I think) but the Gaper clams are not much deeper - siphon is about 4 inches long and the shell another 4 inches approx so 8 - 10 inches below the surface. It's very easy to misjudge the angle and spear the shell, but practice makes perfect...