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It could also be a young P.interjectum perhaps, looking at it. The only reliable ID is with a microscope really.
Ceri, thanks for the link. You are right, of course, to say that microscopic examination is necessary to be sure of correctly identifying Polypodium spp. But I am happy with the P. cambricum ID. The bit in Stace about the relative length and width of leaf-blades is a good guide in my experience. Also this fern has been recorded previously from the stone walls in the area. Don't think it is a young frond as there are plenty of sori on the lower surface. An opinion from a fern specialist would be welcome.
Lat/Lng: 50.4064, -4.2099
OS grid ref: SX430586