I actually didn't realise it was crayon until it rained for a few days and the master piece remained. I did a quick google search and many pages suggested magic eraser. Being such a large area i knew i would go through packets to erase it all. I decided to try the other popular suggestion. . .WD-40
I did a test patch first as i wasn't completely convinced this was going to work. I sprayed it on and wiped it away with a damp cloth. To my surprise it worked and it worked well
I sprayed a patch & wiped it down and kept going patch by patch. Once i was done i gave it a quick hose down and ta-da clean chalkboard ready for a chalk (not crayon) masterpiece.
Looks bit patchy as it was still wet when i took the photo