Error when posting – php glitch??

When posting or editing and saving the post I get the following error:
Warning: Cannot modify header information - headers already sent by (output started at /homepages/path/wp-admin/custom-header.php:333) in /homepages/path/wp-includes/pluggable.php on line 390
when I post.

The actual post is OK.

The site seems to work OK. There is just this error at the back end. Any thoughts?

Thanks as always
Giles