Phil

Your problem is rather different. You are using the tables-based Classic template (not recommended unless you are running a pre-1.3 version of Zen Cart). Your logo is ocntainined within a table cell and in the same row and to the right is another cell. Although that cell is empty, it is defined to have a width of 35% and so pushes back against the cell containing the logo preventing it from centering.

I suggest that you go to your includes/templates/classic/common/tpl_header.php file and remove the redundant cell.