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Based on the location, this is probably H. oculata (NBN at least only shows oculata in south Wales) but Henricia sanguinolenta is also found around the UK (including far west Wales) and I read that it can't be separated visually as colours/form are quite variable. I photographed a Henricia in Scotland with a very orange underside recently which is more likely to be sanguinolenta based on distribution, but couldn't rule out oculata http://www.ispot.org.uk/node/277552
Nick Upton, naturalist and photographer.
You're right & I should have pointed it out (Marlin being down's my excuse). Habitas http://www.habitas.org.uk/marinelife/species.asp?item=ZB1660 says H. sanguinolenta can only be distinguished from oculata by"the very fine dorsal spinelets which have 3-6 glassy points".But my Collins has H.s. as subarctic, ranging S. to NE England & NW Scotland so Barry island looks a safe bet.
Lat/Lng: 51.3888, -3.2792
OS grid ref: ST110663