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Thanks for the comments. From Stace's key I had narrowed it down to prostrata, glabriuscula or laciniata. I settled on laciniata mainly from the pictures of bracteoles. Stace's fig. 10 is rather more transverse than my specimen, but it has the large tubercles and looks to be split to the base of the widest part of the bracteole, as in mine. The other fruit that is quite a good fit is littoralis, but the leaves are wrong. I've added a some more photos that I hope will have enough detail for you to confirm glabriuscula.
Thanks to Ivor Rees, who has confirmed this is A. glabriuscula.
Lat/Lng: 53.1794, -4.4577
OS grid ref: SH358675