No interactions present.
from the side and underneath.
We also need to know the size.
Littorina 15mm, Euspira catena 30mm.
Have a look at the marine snail shells at http://www.conchsoc.org/groupbrowser/Marine%20snail
There more to this than I'd first seen.
First, Littorina mariae is now L.fabalis.
I am certain it is not either of those.
This picture looks like E.catena (which I have never knowingly seen) but the size difference is quite significant Keith.
If you want a real challenge, go here
Lat/Lng: 52.8788, -4.4289
OS grid ref: SH366340