XML and HTML Sitemaps

I definitely have a XML sitemap, but according to Google Webmaster this is my one and only sitemap. I thought I also had a HTML, but it appears no longer to be the case. I use Yoast plugin for SEO. As you can tell I know nothing about this, so looking for some advice please. Do I need both? Will it help with indexing and SERPS results? We are a home insurance brokers in UK. A competitive field.