What is this plant in a wall?

What is this plant in a wall?

Observation - What is this plant in a wall? - UK and Ireland. Description: Plant with runners and lots of small green leaves growing in crack in stone wall. One of many of the same kind. Neither garden nor park but tall urban wall where this (and other plants) have taken root. The leaves have a very slight gloss to them an

Plant with runners and lots of small green leaves growing in crack in stone wall. One of many of the same kind.

Neither garden nor park but tall urban wall where this (and other plants) have taken root.

The leaves have a very slight gloss to them and are about 1cm - 1.5 cm.