Thanks for answers. Have tried both and even went round every single block with a scraper! Maybe I made them TOO clean or just the damp weather has made the problem worse. Will give both remedies another go!
Soda chrystals 80p bag from supermarkets.
Sprinkle over moss, then sprinkle it with water. After a few days it will turn brown, and you can use a stiff brush to remove it.