The relevant guidance from OFGEM that the FiT licensee (EON in this case) must follow is available here:
https://www.ofgem.gov.uk/publicatio...-electricity-suppliers-version-6-january-2014
The relevant parts say:
4.2. FIT Licensees are required to check energy efficiency requirements for every solar PV installation with a DNC up to and including 50kW, including extensions, with the exception of stand-alone, with an Eligibility Date on or after 1 April 2012.
4.5. The EPC of level D or above must have been issued by the Eligibility Date of the FIT installation. Any installation that has not acquired an EPC of level D or above at or before this date will receive the lower tariff.
4.16. Where the energy efficiency requirement applies, the EPC (original, or copy) should be submitted during the application process along with the relevant declarations.
4.21. The EPC information is lodged on the Landmark database and can be accessed here for domestic dwellings:
https://www.epcregister.com
4.23. EPC information can be accessed from the EPC Register by someone who has either the relevant Report Reference Number (RRN) or the address of the property.
4.24. FIT Licensees should check the following criteria against the EPC provided by the FIT Generator:
- The unique RRN of the EPC
- The address and postcode of the building assessed
- The date the certificate was issued (and that it is valid at the time of the Eligibility Date of the FIT installation)
- The “asset rating” to confirm that the building has been rated level D or above
The customer should point out to EON that under the terms of 4.24 that the incorrect EPC supplied was not valid at the time of the Eligibility Date as it had been superceded by the newer certificate.
If EON do not accept they have made a mistake then follow the EON complaint procedure and, if required, follow that up with legal action.