Pete on "Why is my xml sitemap throwing up this error?"

This is the error message when I try to view my xml sitemap at http://www.byfordwa.com.au/sitemap_index.xml

I'm using Yoast SEO but it's the same error message for the other popular xml sitemap plugins so I figured it's not an issue with the plugin.

This is the text of the error message...

XML Parsing Error: XML or text declaration not at start of entity
Location: http://www.byfordwa.com.au/sitemap_index.xml
Line Number 2, Column 1:<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//www.byfordwa.com.au/main-sitemap.xsl"?>
^

Here's a screenshot of the error message...
http://snag.gy/yZyB5.jpg

Thanks for any help :)