red feathery seaweed
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Bunker et al in the Seasearch Guide to Seaweeds list two other species that are similar; Laurencia obtusa and L.pyramidalis. The fine details that separate the species can't be seen on the picture. I can't be sure this is the correct ID.
By the way the taxonomists seem to have changed Laurencia pinnatifida to Osmundea pinnatifida.
It might be Plocamium cartilagineum as I can see comb-like bits at the end of some fronds, but I don't have a seaweed guide anymore unfortunately so I'm not really sure.
Not much to go on but there are glimpses, via the original, of one-sided fronds. In any case Rosie (Nature Girl) is on the ball.
The resolution is poor and the specimen well battered but I really think it is Plocamium cartilagineum but it cannot be easily separated from P.nanum.
Worth reading comments here
Lat/Lng: 50.7624, 0.1509
OS grid ref: TV517980
Cuckmere Haven - marshland with river Cuckmere running through it to the sea - several little lakes and stream running off.