There are three sub species of Asplenium trichomanes, ssp trichomanes, ssp quadrivalens and ssp pachyrachis. A trichomanes ssp quadrivalens is the one most associated with walls and found throughout the UK, while ssp pachyrachis is associated with limestone rocks and ssp trichomanes is found on non calcareous rocks and occasionally walls.
Subspecies pachyrachis may have projecting basal lobed (hastate) pinnae and toothed end wavy margins. This is the rarest plant in this area of Wales.
Subspecies trichomanes may have round to almost circular pinnae that are distinctly stalked. A more upland plant than the other 2 subspecies.
Subspecies quadrivalens has more parallel sided pinnae than ssp trichomanes and on average larger pinnae than trichomanes. The most common subspecies found on walls.
So A trichomanes ssp quadrivalens is probably the most likely identity of the plant growing outside my front door although a lot of the diagnostic features appear to be more comparative, for example, with reference to ssp trichomanes;
‘Pinnae looking more widely spaced and narrower than (b)…due to them curling under longitudinally and sometimes turning up at the apex. generally fronds with a more delicate appearance than (b)‘(Hutchinson and Thomas 1996)
Although they can be determined by looking at the shape of the pinnae where these , a more reliable way is to measure a sample of spores. (ssp quadrivalens has a spore size of 24-43 microns and ssp trichomanes is 29-36 microns) However even spore size does not seem to be a definitive way of determining the sub species. See Stace (1997) page 24, Hutchinson and Thomas (1996) p 95.
Therefore if there is any doubt in the field, and most of us do not have time to check each plant or to measure average spore length, it may be better just to say the plant is Asplenium trichomanes L (in my view!).