How wrong is the Reg. No.? If, for example, its a letter O instead of a zero 0 an adjudicator may consider this to be “de minimis non curat lex” (the law will not concern itself with trifles) and reject your appeal.
With the greatest respect to Bald Eagle, I don't think that the model of the vehicle is mandatory, so a procedural impropriety would not occur.
Some Councils (I don't know if Medway is one of them) allow you to view the PCN Evidence they think proves the contravention online. If they do, try entering your correct reg. no. and PCN Number into it and see what comes up.