GlennB, you might find it useful to read the link from Bob on another thread: https://goo.gl/t5uyWF
Not the first article but the one further down about obtaining Italian citizenship. The people writing the article experienced problems with UK documents like birth and marriage certificates being accepted by the body responsible for deciding on nationality. The British consulate stated that they could not do this BUT they could provide a list of lawyers who are able to certify that documents are valid and those lawyers also have a letter from the Embassy confirming that they are authorised by the Embassy to carry out such a task. Most of the relevant information is in the last large paragraph on Page 5. You could try asking the British Embassy if they have such a system in Greece and if not why not? I realise the situations are different but the problem seems to be very similar.
PS The link is to a pdf document so it might simply appear in your download folder rather than open directly.