Hi Fraoula,
I don't know whether anyone is organising marriages for tourists in Plakias now but they weren't in 1997 so we got married in Agia Gallini, about 45 minutes drive East of Plakias.
Here is a link to some information I posted about it.
If you want to marry in Plakias it must be possible, especially as you speak Greek. Perhaps
Ali &
Dave (the Brits who run the Agora mini-market almost opposite Anso Travel) can put you in touch with the local Mayor?
I don't know what the local costs would be or how much notice is required by the local registrar but I can tell you that you'll need to get your birth certificates translated into Greek through the embassy in London. If I remember correctly that took several weeks and cost about £40 each but that was 7 years ago.