I'm afraid I've been a right numpty and locked myself out of my own WordPress, which is installed in a "/blog" folder off the root within my wider website. I was trying to work out why my "blog.raygun-gothic.net" subdomain was not re-routing to the blog properly, and so changed the URL and installation folder fields on general-options.php from the old "http://www.raygun-gothic.net/blog" to "http://"blog.raygun-gothic.net". I think one of these also had a trailing "/blog" left on it - a casual error.
Unsurprisingly I have now obliterated access to my own WordPress blog, including the admin. page from which I could change this back. I've tried searching the blog directory source code for any mention of the URLs so that I might change them back manually, and there are none. How can I switch the URL and installation folder values back?