For those of you who have pressure washing as an add on to your business, can I recommend you try ALGEX from Restormate, it's basically an algae eater, removes the stuff from Stone, brick, wood etc.
I used it on my rear garden last friday, which was green, by Sunday it had all gone. We've also just used it on a big decking area for Alnwick Gardens and secured a 3 year (3 times per year) contract on it. I have to say I'm pretty impressed with it.