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It looks like Balanus perforatus - according to Hayward and Ryland: "purple to pink...wall plates slightly ribbed...orifice usually very small...often abundant in SW England". But you need to add the identification so people can agree with you!
Thanks for your help :)
B.perforatus is common in the low shore in SW England where they are usually the commonest intertidal barnacle with a calcareous base. Given the size of the Spirorbis worms alongside these are probably last years's settlers and are growing quickly. Its worthwhile noting that normally individuals seen are considerably larger and join together to form a matrix as in other acorn barnacles.
This is a species that appears to be spreading its range as the climate warms.
I was going to add a second identical Id to see if this would trigger a Group change (to Invertebrate) but there is little point as the original agreements would hold the first ID as Likely.
Then, I discovered that the name should be Perforatus perforatus - see http://www.marinespecies.org/aphia.php?p=taxdetails&id=106219.
Of course, the UKSI dictionary, which is out of date (by a long time), would not support the 'correct' name, so I relented!
THREE MARINE PROJECTS
Lat/Lng: 50.1, -5.1
OS grid ref: SW8032