OK
I have the technology, I have a nice 2 square beach with two empty spots behind (I'm assuming that's what the 4 space requirement means) within the city area, I have the gold etc. required, and still can't build a harbour.
Anyone have any suggestions?
Some of this is just a touch TOO arcane.
[edit] answered my own question again 
It was the city being too close to another to have buildable tiles on the beach. So, next question, can I destroy one of my own cities (he he)?