Very fine filament seaweed, photo shows an area approx 35x50cm
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Chaetomorpha is a genus which has unbranched filaments, and I think that, when I zoom in on your original photo, these filaments are branched. Beyond that, without microscopic examination, I would not care to speculate. Perhaps, as the observer you have noted other features which could aid ID.
Lat/Lng: 49.9136, -6.315
OS grid ref: SV903104