class CustomPage_Controller extends Page_Controller{
   function EditPage(){

      //this is called in a url, mysite.com/mypage/EditPage

      if(!$this->UserIsAdmin()){
         Security::permissionFailure(null);
      }
      
      $fields = $this->EditableFields(); // correctly returns fields needed in form
      $actions = new FieldSet(
         new FormAction('UpdatePage', 'Update')
      );
      $required = new RequiredFields(
         "Title",
         "Content"
      );
      $form = new Form($this, "EditPage", $fields, $actions, $required);
      $newdata = array(
         'Title' => 'Editing '.$this->Title,
         'Content' => '',
         'Form' => $form
       );
      return $this->customise($newdata)->renderWith(array('Page','Page'));
   }
   function UpdatePage($data, $form){
    // ... code here, but this method never gets called...
   }
}