jason404 on "[Plugin: WordPress SEO by Yoast] Sitemaps not working"

I noticed I am getting crawl errors in Google Webmaster tools, and found that the WordPress SEO sitemaps are not working properly:

http://www.ianstuart-bride.com/sitemap_index.xml
http://www.ianstuart-london.com/sitemap_index.xml
http://accessories.ianstuart-bride.com/sitemap_index.xml
http://accessories.ianstuart-london.com/sitemap_index.xml (not currently accessible)

This error message shows when viewed in Firefox:

XML Parsing Error: not well-formed
Location: http://www.ianstuart-bride.com/main-sitemap.xsl
Line Number 1, Column 699:

W3 Total Cache, to see if that was the culprit, but it isn't.

What's going on?

Thanks

https://wordpress.org/plugins/wordpress-seo/