Certificate of Tax Residence for Greek authorities

I've went through the process of issuing a certificate of tax residence through HMRC, but since I do no have any income to declare from Greece to the UK, I receive a Letter of Confirmation instead. Despite the letter mentioning that this should be accepted from the Greek authorities, apparently this is not the case. They still claim that they need a proper certificate. Is there any update on that from the HMRC? Can I somehow enforce a Certificate of Tax Residence to be issued via my application? Thanks in advance.

Hi,
A certificate is only applicable when foreign income is received. 
If the Greek authorities will not accept the letter of confirmation, you will need to return this along with their rejection letter for an appropriate response to be issued.
Thank you.