Confidence: I'm as sure as I can be.
Notes: The Handbook of the marine fauna of NW Europe has female 12mm, brown... carapace poorly calcified. Parasitic in ..lamellibranchs(=bivalves).. commonest in Pinna, also in Modiolus & oysters. Also in tunicates: Ascidia mentula & Ascidiella aspersa. SW Brit,scarce.
Mine was found in a large seasquirt, hence the ID (Pinnotheres pisum it seems is only found in shells, & accordg to asturnatura.com sometimes in echinoderms or tubeworms
NB the Pinna is the large,rare & protected fan shell