Finally got round to changing over to css buttons. Took me a while to work out why my customisations of certain buttons were not taking affect.

Turns out I had changed most of the *.gif buttons and language defines to *.png but had forgotten about this line of code in function zenCssButton in \includes\functions\html_output.php

Code:
   $button_name = basename($image, '.gif');
Changing .gif to .png fixed my immediate problems, however, it seems that I may have to change this code and all the .png buttons and language defines back to .gif because there is currently no provision for mixed image types (eg gif, png, jpg). Sigh!