Finally, there was a problem with the icon for the xowikiimage plugin.
To get the icon to display you have to add a 15 x 16 pixel icon (i.e. xowikiimage.png). The path specified in the xowikiimages plugin is not consistent with the ckeditor standard.
To be standard, add an 'images' directory under plugins/xowikiimages/ and place the gif in there.
Edit plugins/xowikiimages/plugin.js to include the icon declaration in the editor.ui.addButton function:
editor.ui.addButton( 'xowikiImage',
{
label: editor.lang.xowikiImage['insertImage'],
command: pluginName,
icon: this.path + 'images/xowikiimage.png'
} );
The problem was that "icon: this.path + 'images/xowikiimage.png'" this was incorrectly located in the CKEDITOR.plugins.add function instead.
Regards
Richard