dont think thats problems is exsisting damp in bricks of that summat salts . any way to the problem at hand that flash band deffo need s replacing only last 12mths any how.
just to keep u up to date folks brett and marshall now do an outside grout
for there sandstone products 8) and a sealer which should be applied before groutin
not in the best of conditions also looks like you have lead soakers and there s not enough cover of lead goin to them , also is there any damp on the perling in pic 3