have you considered contacting the financial ombudsman service
www.financial-ombudsman.org.ukif ok pay's financial services partners have offices in the UK, then hopefully they should be covered, though not sure, as I don't think OK pay is in the uk, but definitely worth contacting the FOS. if Ok Pay isn't within their remit they should be able to advise you which ombudsman in which country it comes under.
if they are in germany, then I think they have pretty good banking regulations and hopefully have an ombudsman. scammers or incompetent, it doesn't matter, you should make a complaint about the distress they caused you and request compensation!! in the UK, using the financial ombudsman is always free for the consumer, not sure about other ombudsmans in other countries