I think it is a Maidenhair Spleenwort (Asplenium trichomanes), I may be corrected though!

No I am happy with your id, That's what I'd say it was. it is a wonderful British native. lucky you.

Thank you Lynn08. I goggled your suggestion and it seems to fit the description. It is growing in the old lime mortar of a wall. It also has the dark stems and leaf shape in the description I read. I have added a blown up portion of the original photo.

Thank you too Sbg. Indeed Lucky me. I love it. I do not know where it came from but I'm delighted to give it house room. I'm hoping to transfer it to a trough. It is evergreen, how lucky is that.

We have a lot of them growing on the walls in this part of Ireland too, I think they look lovely!

A present from Ireland then brought in by the west winds. I can't say I have seen one like this around here but I suppose they are.

I hope it will like growing in the trough, only they seem to like growing out of tiny cracks in walls. I'd be inclined to leave it where it is, & hope some more will appear.

I would leave it in the wall too as that's where they put themselves naturally. Loads round here, and all in walls.

Thank you Feverfew and Steragram. I wondered if it would grow in old lime mortar in a small wall in a trough?
I went looking for it and found it being recommended for troughs on the SRGC website. I even found a supplier.

it looks a bit like this
Ceterach officinarum

it too grows amongs rock cracks

Thank you Saranfiljka. I googled your suggestion and it came up as syn. with another Asplenium. It has green stems and mine has brown stems so I think I have Asplenium trichomanes. Members are really good at coming up with suggestions and all comments are gratefully received.