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Many thanks for this id - a new one to me!
... bridge, locks, retaining walls, aquaduct, viaducts, ...
It also turns up on walls elsewhere, but in my area at least there's a definite tendency of association with the canal network in particular.
Other ferns that you find in the same habitat are maidenhair spleenwort (Asplenium trichomanes), (depauperate) hartstounge (Asplenium scolopendrium) and more rarely black spleenwort (Asplenium adiantum-nigrum) and rustyback (Asplenium ceterach). Depauperate male and buckler ferns also occur, but are difficult to identify at that size.
Thanks again for your help and useful information. I can manage to id Maidenhair Spleenwort and Hart's Tongue, but I haven't come across the others you mention, so will keep a look-out!
Lat/Lng: 53.609, -2.30277
OS grid ref: SD800125