My first question would be, "Are you happy with the bike?"
Mis-selling aside, do you actually like the bike you have been riding for the last 3k miles. Would you actually want another bike?
If you are happy with it - then I'd angle for compensation equal to the difference in the price you paid when compared with the price you would have paid if sold it as a Cat C or D bike.
Though how exactly you go about this I have little clue I'm afraid
- my first step would be to write down a timeline of the history of the bike that you have evidence for. Then call Citizen's Advice.