"You want to be carefull about not letting to much on the sand fill to wash out."
Why this advice? Sand hasn't been mentioned as a fix.
THere are flexible sealants that come in a caulking tube that will bridge fairly wide gaps that would work for this I would think. Typically need to fill the void with foam rod (flexible foam spaghetti-like strands that come in various diameters) and then apply the sealant. HD has one type; there are likely more and better choices at a local masonry/building supply dealer.