my thought
block the down pipe with a tennis ball or rag divert the down pipe into a bucket iff possible assuming its a terraced house block the gutter at each side off the tiles
paint on nitromors water based onto the lead and tiles concentrating more on the lead and seeing how gravity spreads it
once fully coated and left to work in accordance to instructions see how the paint comes off and act accordingly
i suspect the tiles may be painted together by cappilery action preventing easy removal and possibly aiding water protection
so slap it on and see
oooo and remove the car or any other painted item from the area[4m away if possible]