This was growing up by the croft we stayed in on the south side of Loch Broom at about 140m above sea level (it felt higher walking up to it!). It's Polypodium vulgare or common polypody, one of our commonest pteridophytes. The underside shows the sori which develop on fertile fronds. They contain the spores and are not covered by a membrane called an indusium which is found in other ferns.
They are called polypody on account of growing from rhizomes which have lots (poly) of apparent feet (pody). They are found growing on walls (like this) and in banks, as well as on trees as epiphytes.