Although the cut-off for the introduction of Euro 6 standards is given as 1st September 2015, there was some flexibility within the rules at the time and the guidance indicates that vehicles sold up to 1st September 2016 may still have a Euro 5 engine, and owners are encouraged to check with the manufacturer for confirmation.

It is possible that the dealer has relied entirely on the TFL site or other ULEZ-compliance service to check its validity and based on the registration it would appear to be compliant, although the V5C indicates otherwise. You should contact the dealer about this matter. The letter of compliance they have provided will certainly help your case, but in the event that they are unwilling to rectify the situation you could reject the vehicle on the grounds it is not 'as described'.