This is what I have:
http://page-test.co.uk/js-test.html

..and it works exactly how I want it to, but I need it to work by doing these two things:

(a) If you see in the change_button function, I have hard coded the 'name_can_vary' name element. I need to do it so this isn't hard coded (so it can work on any page/form).

(b) I don't want to change any of the HTML, and I only want to change the code used in the change_button function (otherwise I will need to change many pages).

Thanks in advance.