That's correct about your signature.
The sitemap just shows google that every page is pointed to from the sitemap with url text that contains the words Website Design - Plymouth, Devon that's why they do well.
What people often missed about pagerank was you could get a PR4 with only having about 4 inbound links the rest was all to do with internal linking.
i.e. All pages should be reached from the home page in under 2 clicks. If your sitemap is reached from the home page in 1 click every other link is always only 2 clicks away no matter how deep it's buried.
Look at the sitemap on my site....
Centrereed's Sitemap
This can be reached from every page via the menu and even reached when javascript is turned off. Try it and turn javascript off and refresh the page you can still navigate the site because it's only 1 click away. Google note these factors and it's surprising how many "fully qualified" web designers miss these facts.