Can there be presently some type of application which could day to day check against an internet site on whether it's been up-to-date?

HTTP headers have an "If-Modified-Since" request, which in turn causes the server to come back merely a "304 Not Modified" as opposed to a whole page. This is often sent by any application able to HTTP comms.

List of HTTP headers

(my Python senses are tingling)

You will get the final-Modified HTTP Header, for instance (C#):

HttpWebRequest request =(HttpWebRequest)WebRequest.Create("http://www.yoursite.com");
HttpWebResponse response =(HttpWebResponse)request.GetResponse();
Console.WriteLine("Last Modified: {0}", response.LastModified);

You could do this this beautiful easily having a cron job, curl/wget, along with a revision control system (for example git or SVN). You'd require a *nix system to make the most of it.

I'm not sure that anybody has packed together an easy application to handle this for you personally though.

Used to do a fast Search and located a minumum of one program but I have not used at all it and so i can't attest to it personally: http://www.google.com/search?q=check+updates+web+site. WebSite-Watcher may be the first result right now for your query (http://www.aignes.com/).