Honestly as simple as your site is, you probably don’t need to go through setting up a subdomain. If you physically move the “Rollerston” folder up one level in your site file structure, It will go from “http://www.harrythefish.com/other%20sites/Rollerston/Home.html” to “http://www.harrythefish.com/Rollerston/Home.html” (though depending on the coding you may need to fix a few file paths in the code).
Then if you rename Home.html to index.html (fixing links to that page) someone could type “http://www.harrythefish.com/Rollerston/” and it would hit the right page.
However, as Ben_Derby said, using .htaccess is really the clean way to do it. It would allow you to set up forwarding from the old URLs so any links you have to your site don’t break. And you can set up other clean URLs like “http://www.harrythefish.com/Rollerston/Contact/” using mod_rewrites. It takes a little bit of know-how, but it’s not a complicated process.