Hi John, its worth pointing out that sodium hypochlorite (bleach) dose not have a long shelf life and will degrade in strength over time, so Domestos even though its 5% active can degrade to 1% or less if it has been lying idle over time, sodium hydrochloride has a much lower content of chlorine and will also degrade in strength over time which sometimes can render it useless if its out of date, we often go to a local water treatment company and purchase 10% active sodium hypochlorite as a secret weapon to solve problems like you have mentioned above, but again if its been sitting on a shelf for quiet a while its useless.