You may be right about the nature of your Derbyshire gritstone @pansyface. Perhaps it's too hard to absorb a surface cleaner like this which needs time to penetrate into the substrate to kill off all the algal/moss growth. I've never heard of gritstone before, it must be a strong building material.
Another thumbs up for Patio Magic, although it's a tad expensive for the amount of Indian sandstone we've got right around the house and back garden terrace.
Not quite as much however as the Black spot remover stuff you see advertised.
Posts
Not quite as much however as the Black spot remover stuff you see advertised.