Growing in moss on, or very close to, the base of a birch tree on Leighton Marsh RSPB Reserve.
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Blechnum spicant would have continuous linear sori either side of the midrib. I think Polypodium is more likely, but as you say it would be better if more of the plant was visible.
... having got to the point where I can distinguish the foliage in the field I had forgotten the sori character for Blechum - in this case it would appear that I've been fooled by the foreshortening of the pinnae.
Lat/Lng: 54.16593, -2.80169
OS grid ref: SD477747