I've decided to bite the bullet on this and give it a go myself.
 
I installed D6 and then uploaded the customised theme with an info file with the customised regions listed.
 
I followed the advice here: http://wtanaka.com/drupal/convert-theme-6
 
and renamed some of the variables throughout the theme. I'm currently stuck at number 5 on that page as I have the following in my custom theme's template.php
 
/**
* Catch the theme_profile_profile function, and redirect through the template api
*/
function phptemplate_user_profile($user, $fields = array()) {
  // Pass to phptemplate, including translating the parameters to an associative array. The element names are the names that the variables
  // will be assigned within your template.
  /* potential need for other code to extract field info */
return _phptemplate_callback('user_profile', array('account' => $user, 'fields' => $fields));
  }
 
When I try to go to user/1 on my new D6 site, I get this:
 
Fatal error: Call to undefined function _phptemplate_callback() in /d6/sites/all/themes/xxx/template.php on line 115
 
and I presume it's something to do with this. The above page says to get rid of phptemplate_callback () but not what to replace it with.
 
I would appreciate any help with this. I think I'm about 70% of the way towards a 6.x theme and this would be another big step.
 
Thanks
 
Neil