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Origin 14 July 2013
Oh yes, I saw this when you posted and MEANT to research then and, possibly, agree. I was then waylaid by the weight of moths (you'll know what I mean!)
So...this cannot be anything other than Sagartia elegans, unless of course you were in Antarctica in 2008!
So you have an agreement.
It IS likely to be Var. rosea (as you had previously intimated and have now cunningly placed)
"...the disc is variably coloured, plain or patterned as in miniata with the tentacles being pink, rose-red or magenta." MarLIN.
I have the 1860 Gosse book to hand. It carries Edward Step's (Yes!) handwritten notes in the margin "Sag.elegans (Dalwell) Var Rosea, Gosse".
Gosse called it simply Sargartia rosea but he goes on, in the book, to describe four sub varieties.
Gosse lists it as present "Known to me.." at Tenby and St.Gowan's (and four others outside Wales).
Your quality picture is a delight - it shows all the detail described by Gosse, down to the "undefined dusky bars..." at the foot of the tentacles.
So there you have it - history!
Many of us forget to agree when we add comment - I'm sorry!
The link (in the next comment) to the Book is still viable well worth a browse - though WITHOUT the Audion Reader!
THREE MARINE PROJECTS
Goodness - this book (the original publication which I own) is online WITH Audio reader.
You may enjoy some of the passages - the electro-voice is distracting.
Lat/Lng: 51.5509, -4.1505
OS grid ref: SS509857