The seller will be responsible for giving you a false description,
even if they believed it to be true.
You can advise the seller that if he does not accept the car back
and refund you the full amount (less any discount for mileage
you have done) you will have no option but to commence legal
proceedings against him. You need to make it clear that he has
acted in breach of contract and has misrepresented the sale. Not
only that, but he has also committed a criminal offence by
selling a clocked car and you may also have to contact the police.
Hopefully al this will make him think again about whether to
accept the return and hopefully this matter will be resolved
without you having to go any further. Should he refuse, you have
the option of going to court to recover the money by submitting
your claim at www.moneyclaim.gov.uk