You need to go back to RBS and point out that they no longer own the debt, and ask them to justify why this has been withheld. Make it clear that the moneys was going to be used to pay down the debt with the LEGAL owners of the debt.
Also let them know that YOU intend to take this further, because as the debt is not owned by them.
However, some debts are sold on a 'buy back' basis if PPI is reclaimed, and if that is the case you must insist that RBS notify the new owners fully that the debt has been reduced by £817.
You must also do this in writing, because if you need to take this further, well lets just say a lot of telephone call never get recorded by RBS when they should be, or rather the recordings get lost!
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Abbreviations used in DQF
My advice is guidance only, if you want the law then consult a lawyer!
(c) All Spelling mistakes are my own design, infringement of them may result me sulking!