This looks to be an example of Littorina compressa
No interactions present.
Goodness Graham, it is more confusing that we thought or deserve!
I hope you are right. Cannot agree yet.
Note I have posted 'my' Littorina saxatilis here -
adds to the confusion.
http://doris.ffessm.fr/fiche2.asp?fiche_numero=1655 example of a grooved periwinkle
Lat/Lng: 51.3836, 1.3217
OS grid ref: TR312701
high eulittoral zone of chalk reef.