Good question! My Greek isn't quite up to advanced standard yet so I didn't have an immediate answer.
But I had a quick look at the FREE lessons from CYBC- see the Free Greek Lessons link on the Learn Greek page - (lessons are only available online though I'm afraid).
Out of the 105 lessons, lessons 76 onwards are classified as Advanced. I had a quick listen and didn't understand a whole lot straightaway so this might be a good place to start.
At any rate, the lessons are free so as long as you have a broadband connection there's no cost, although there are books to go with each block of lessons if you want to see the written Greek too.
My husband just bought one of the books for the beginners section and it cost 17 Euros. If you think they are advanced enough, let us know by adding a comment.
If I come across anything else in the meantime I'll let you know.
