Oak spider

Yes, both aculepeira ceropegia and aculepeira armida are very common. They are just difficult to distinguish sometimes.

Many thanks Ioannis, much appreciated. I didn't expect it to be a new species as it is very common on Ydra.

There are two species of Aculepeira that can be: Aculepeira ceropegia (from greek κηροπήγιο, candelabra the shape of the motif on the abdomen) and Aculepeira armida which is a little more orange.
Aculepeira armida is found only near Mediterranean and Aculepeira ceropegia is found all over Europe. There is a third species, Aculepeira carbonaria but it's found in higher altitudes and rocky areas so I don't believe it's the one in Spetses island.
From the light and shades, I can't say if it is the Aculepeira armida or the Aculepeira ceropegia you have found there.