Here in the U.S. there are several products at the garden store one could use to get rid of the ugly stufff. They products are designed to eliminate moss growing in the lwan but won't harm your roof. It comes in largish bottles that are designed to attach to a hose end (ya'll call it a water pipe, no?)
Then, slip thin strips of copper or zinc under a row of shingles near the top of the roof with about one or two inches showing out the bottom of the shingle. The rain running down the roof will leach tiny elements out of the copper or zinc and prevent further encroachment of the green slime...