[Plugin: WordPress SEO by Yoast] Problem with my sitemaps – google says they are html

This is what my sitemaps look like.

This XML Sitemap Index file contains 3 sitemaps.

Sitemap Last Modified
https://www.bournemouthguitarlessons.co.uk/post-sitemap.xml 2012-07-07 12:22
https://www.bournemouthguitarlessons.co.uk/page-sitemap.xml 2012-07-07 20:48
https://www.bournemouthguitarlessons.co.uk/category-sitemap.xml 2012-07-07 12:22

So i submitted the index file /sitemap_index.xml and this is what happens in Google Webmaster tools.

1

Errors
Sitemap is HTML
Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.
2
Sitemap: /category-sitemap.xml
–
Jul 7, 2012
Sitemap: /post-sitemap.xml
–
Jul 7, 2012

Can somebody shed some light on what happened please? I always had just 1 sitemap and suddenly it split into 3 so do i submit the index or do i submit all 3 sub sitemaps individually? I’m a bit lost and confused.

https://www.ads-software.com/extend/plugins/wordpress-seo/