Like half of Ireland we optimistically put down an Indian sandstone patio, porous as swiss cheese and so now covered with black spots, which have turned out to be lichen. When I went in search of help, an unhelpful man man in the stone shop said that stuff isn’t suited to an Irish climate and we should never have put it down in the first place. Well it’s there now.
So I sent for another man and he’s brushing swimming pool chlorine into it.
See here for instructions if your idealistic patio life for those 48 hours each summer is blighted by black spots on your indian sandstone.