You need to write to one of the credit-rating agencies (e.g. Experian) and explain the situation and state that the record is erroneous. Assuming that it is an error (i.e. you haven't miscalculated) then they have to remove it.
However, your story is not too clear: You say you got "confirmation that DC would not be chasing the money...." What money? You said the account was clear. Are you sure that there wasn't £77 outstanding but the lender decided not to persue you for it (i.e. you had paid enough of the debt not to make it worthwhile / enforceable)?
If you did leave £77 owing then it's a genuine mark-down on your credit score and there's not much you can do about it.
Using those high-interest lenders does your credit rating no good even if you pay it all off. More mainstream lenders will shy away from users who use the high-risk lenders.
Current: 2013 F10 520d M-Sport, Professional Media, Visibility Pack, Reversing Camera, DAB
Previous: 2011 F10 520d M-Sport, 2008 320i M-Sport Coupe, 2002 325i, 2001 318i valvetronic, 1998 318i, 1996 525i, 1990 Porsche 944S2