Getting Married in Greece, I would strongly advise that you initially contact the Greek authorities and determine exactly what documentation you need. (if you don't have the correct documents - you will not be getting married!!)
If you are two British People getting married in Greece, you will need a Lex Loci (Certificate of Non-Impediment) available from the Register Office in the district where you live. You will both have to attend your relevant Register Office and give your legal notices, you will need to check the fee for giving notice, and you will need to take the relevant documents with you, i.e. your passports, proof of address, any previous marriage or name change documents. Also you need to know the district and country of where you will be married. In Greece, Certificates of Non-Impediment are valid for 3months only, from date of issue which is 16 clear days after giving notice.
If you are a British subject marrying a Greek National (or any other national), then you only will have to do the same as above, but the paperwork will be issued after 21 clear days. The other party should contact the relevant authorities, in Greece or their country of nationality.
You will most likely have to have the documents stamped by the Foreign and Commonwealth Office, and maybe also translated into Greek. You will also need a copy of your full birth certificate, (not a photocopy) I have been told that the Greek authorities keep all the documents submitted. You can obtain full birth certificates from the Register Office in the district where you were born, or from The General Register Office. You will also need original copies of any name change deeds you may have had done or any divorce documents.
Also you must also check the residency requirements. In some countries you must have been there for a specific period of time before you are able to get married.
BUT - most importantly: check the requirements needed, first with Greek Authorities, either in Greece where you are getting married or the Greek Consulate in London (not by hearsay or travel companies or even wedding planners!) The legalities of marriage change from time to time so we advise to check with the relevant authorities first, we are not always updated with changes immediately.