It seems to be the thing to do, to have your roof cleaned and then possibly paint it black. But it's surely a dodgy thing to do as the water can get under your tiles during the washing, and I've been told the cleaning can render your roof porous. Oh what a great idea let's all do it! 

When I had the quote for the guttering I did ask him about roof cleaning because the survey had suggested that the moss might need to be removed. He just said no, don't do it, it can cause problems.