This is one of the basic forms of life, including liver wort, and lots of elbow grease required to kill it off. On a dry day you can 'blow torch' it. Super strength glyphosate, in dry weather, can also have some effect. In early spring it lifts more easily out of stones, so you can try some scraping out before it flowers. Jeyes fluid is very good, as is diluted bleach, but be careful about drainage from the cobbles into any precious nearby plants or shrubbery, or pets, until it has dried off. I have also been told it is edible - a bit like seaweed - but never tried it, though I do like deep fried seaweed!!!!

The bio-degradable path/patio cleaners claim to get rid all algae, moss and lichen, I use Armillatox but there are several to choose from.

Thankyou both for your advice, am now armed and ready to do battle against this horrid stuff!!!.