Georgian era ended in 1830 and this means Peter meant that candlesticks came to the family during Victorian era and perhaps John should have found out details about their previous owner. Still there is no doubt that he was totally misled as he trusted Peter.
This contract could be set aside because it is a misrepresentation1 from one side and a mistake on the other. Misrepresentation was done in clear words, with a special knowledge2, over dinner at Peter's residence by Peter, upon which the entire Contract stood. John has to prove that it was not a matter of opinion regarding the nature and originality of the candlesticks, but was an intentionally made statement, because if it was a matter of opinion, misrepresentation does not come into picture3. John can prove it easily because Peter did not refuse the 7000, which is exorbitant for a piece as recent as Victorian. Peter also has a possibility of showing that he genuinely believed that the piece was Georgian; but such possibility is remote.
John's mistake could be argued as 'being led to it' and not an incorrect understanding and this can be a ground to invalidate the agreement.