We've tried every year Jimmy with power washingand various chemicals .It seems to be a never-ending job...
apparently there are particular slabs which don't attract the spots.

Same here, Jimmy,but ours are the slabs,used as a circular feature,and all the white marks are said to be Lichens,which are a natural occurance with this type of slab.. No amount of cleaning will shift them,as we have tried.. We just have to live with them..