I usually use Cementone Brick and patio cleaner as i find that is quite effective. Results can vary depending on the strength and how diluted the acid is. If there is alot of efflorescence, then i would use it undiluted. BE CAREFUL however. I always use pvc gloves and eye protection when using it as it is acid based. On the bottle it always advises to test it on a small area first to see how well it does and I too would advise the same. It can be purchased from any DIY store for under £12.