In article , Tumbleweed
writes

Once its cleared you'll have to keep it clear and not let it build up. Buy
your own high pressure cleaner (watch out you dont wash out the mortar, you
could also try dilute jeyes fluid but it stinks!

Jeyes fluid is now banned.

We just use a power hose on ours about twice a year.

Unless you are opposed to using chemicals, use something known as hypo -
I think its correct name is sodium hypochlorite, but don't quote me. A
very dilute solution watered on to the drive should keep it clear of
moss.

--
Jane Ransom in Lancaster.
I won't respond to private emails that are on topic for urg
but if you need to email me for any other reason, put ransoms
at jandg dot demon dot co dot uk where you see