Anyone know if a header has been ceramic coated can it be done again??

Yep, sure can.

While on the subject, who can recommend a good ceramic coater? north of sydney prefered

I havent used these guys but there is a place called procoat on the central coast. They do powder and Im told they now do ceramic also.

1, 1 Clare-Mace Crescent, Berkeley Vale NSW 2261
Phone 02 4388 9946