What is Request.UserHostName actually returning and is it possible to convert it to something that I can get a host entry for?

The host name of the client is not normally known because it is not transmitted at the HTTP level. The server cannot know it. Look at the HTTP requests with Fiddler to see for yourself that there is not a lot of information available to the server (and the client can forge all request contents of course).

Use the UserHostAddress property to get the IP address. That is the most you can reliably find out. Once you have that you can try to reverse the IP to a host name but that is not always possible.

I have a more specific answer to your question. By examining the source code for HttpRequest.UserHostName here, I found that it maps to a IIS server variable named REMOTE_HOST which is described here. The property will return the IP adddress of the client, unless you have configured IIS in the way described, in which case IIS will do a reverse DNS lookup to attempt to return the name associated with the IP.