The most likely candidate is Asplenium trichomanes - thrives in limy soils, but is quite adaptable in terms of soil ph. Its an evergreen fern, reaches about 10 inches by 8 wide.

I used to live not far from Stocktonbury gardens and visited them on numerous occasions. They are beautiful but I was sad to see their use of slug pellets.

If it likes limey soil that could be why its happy tucked under the concrete! Wonder why one of the fronds just left of centre is different from all the others?

Now you mention it, Steragram, there does appear to be something else in there - the pic's not very high resolution and does not magnify well, so I'm not sure what it is.

Could it be a Blechnum spicatum? Bit unlikely, I've only seen them in shady places.

Could be a clipping from a nearby conifer?

Lol, I suppose it could!