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...it looks as though it's been painted on!
Sorry, can't help with an ID, but that's a lovely photo.
How to take close-ups with cheap phone and hand-lens:
Field Guides for Budding Botanists:
or Peigne des Néréides in French, is what this is generally called.... though there is a long discussion at http://www.ispotnature.org/node/374537
about the current taxonomy of this seaweed.
If you post an ID you will get agreements.
I still have this rather lovely stone, so will admire it all the more now for knowing what
was living on it
Lat/Lng: 50.6313, -2.3522
OS grid ref: SY751813