Hi Darren,

Welcome to the forum! If you or your developer places the below at the top of your .htaccess file this should fix the problem:

# WordPress SEO - XML Sitemap Rewrite Fix
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^sitemap_index\.xml$ /index.php?sitemap=1 [L]
RewriteRule ^([^/]+?)-sitemap([0-9]+)?\.xml$ /index.php?sitemap=$1&sitemap_n=$2 [L]
</IfModule>
# END WordPress SEO - XML Sitemap Rewrite Fix

Hope that helps. Let us know if you have any other issues or questions.

Best Regards,
Dave