With crevices 2 inches wide, you'll want sand and/or gravel between
the slabs, flush with the slabs, so people don't roll their ankles on
the edges of the slabs. With very narrow crevices you can have the
soil recessed, and a fragile plant that is protected in the crevice.

ld0121 wrote:
Birmingham UK

That's coastal temperate, cool and wet year round. Similar to Pacific
northwest in North America. Moss would do well. Go out scouting for
some in the neighborhood, and make a mess as Billy described (I would
do it in a bucket) and smear it where you want the moss to grow.

Another crevice plant that works well is something sold as "Irish moss"
in the US that is not a moss but an angiosperm, species Sagina subulata.
It likes sandy soil and copes better than moss does with being stepped
on.

You might also try a very short variety of turf grass. One that never
grows more than 1 or 2 inches tall, so you don't ever have to mow it.
These are grasses used on golf courses. However, this may involve
more weeding than you care for. In a highly urban (paved) environment
where natural sources of seeds are limited, weeding would be minimal.

Una