I have been looking to get this kit but there doesn't appear to be anywhere outside the USA to get itAnother tester just had success on copper stains that had turned black using muriatic acid with the tool I will link below...this is likely only a suitable approach for plaster...and would still require some metal sequestrant, i suspect...
Not especially cheap, but it seemed to work well for him:
Amazon.com: Purity Pool UAW Underwater Acid Wash Set: Patio, Lawn Garden
(Since the MA worked on it.)
I have seen it on Ebay for about $140 when everywhere else is about $70 but they don't ship to the UK
Does any one have knowledge of a retailer over there that can or will ship here