@picks44 @Jack
Looks like you're both using the WordPress Multilingual Plugin (WPML)
/wp-content/plugins/sitepress-multilingual-cms/
There's a known bug mentioned here http://wpml.org/forums/topic/yoast-wordpress-seo-sitemap-xsl-error/
Also maybe worth checking some of the other topics http://wpml.org/forums/topic-tag/yoast/
With v1.4.19 onwards, Yoast made a lot of code changes inline with latest WordPress best practices.
Sorry I can't test the fix from the forum - commercial plugin