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Its a scale worm but I wouldnt be able to help with the species. They are possibly quite difficult to seperate if my memory serves me correctly- from a comment on iSpot earlier this year.
Jack, Nicky's ID is very close and Cathal's caution is very worthy.
I'd put money on this being Harmothoë imbricata (looking at the original and brightening it a bit); there are a few other posts of imbricata and I suspect the very dark form is quite rare. You won't get many agreements but you could try a new ID and tag it Marine (you have Coastal/Rural). It might then get some more attention - only might!
Now go here -
THREE MARINE PROJECTS
Lat/Lng: 50.143, -5.068
OS grid ref: SW808314