Claim for the application of the double taxation convention between Greece and UK

Hello, I am a UK resident for tax purposes and am due to receive a one-off payment from a Greek institution. In order to process the payment, the Greek institution has requested that I fill out the following form: https://www.lawspot.gr/sites/default/files/annex_files/other/aitisi_pol-1249.pdf They have asked me to send the completed form to HMRC for stamping and signing, and then return it to me. Could you please confirm if this form is acceptable to you? Additionally, could you let me know the correct address to which I should send the completed form? Is there any other document I need to include?

Hi,
Yes, if you send the completed Greek form to HMRC, Self Assessment and PAYE, BX9 1AS, with a short covering letter that includes your National Insurance number, we will print a copy, sign it with a wet signature and date stamp it and return it to you with a covering letter.  
You can check when to expect a reply here:
Check when you can expect a reply from HMRC
Thank you.