I have actually found the right answer to this one, but wonder if anyone else had a bit of bother with it.
28D Greek island in the northern part of the Aegean Sea, chief town Kastron. (6)
The point I'm making is that everywhere I've looked up, including Wikipedia, the chief town is given as Myrina, and also on GoogleMaps it's the biggest, while Kastron is a tiny village to the north, hardly on the map.