Is this a common type of extension?

Hello? Of course it is a common type of extension, read the adress bar in your browser to see further examples.

Can anyone shed any light on the features of an address that may end like this?

The information is *probably* being passed to a CGI script. That script is *probably* doing something with it. The script may be in *any* languague, probably not perl. Short of hacking the server or finding out what software it is running and groking the source there is no easy way to know. If you have 1/2 a brain you can look at sequential requests and see what happens to ch= as you do different stuff. This will let you guess it's function with reasonable certainty if you are clever enough.

I have spoken to the people responsible for the maintenance of the website, who do not know.

You have spoken to the wrong people.

Is is possible this could relate to some kind of search function?

It is possible this could relate to anything. It is probably a session tracking ID or a document identifier if you want the two most common posibilities. A search string would look roughly like ?q=find%20this where you searched for "find this"