Thanks so much for the quick response!
I have added this as the first line to my theme's header.php file, and it is not redirecting when I test on IE8 (any/all pages in http://bigmoves.org/staging/ ).
I also tried attaching it to routine functions in functions.php, and also giving it its own unique function name and calling that in header.php. Nothing seems to make a redirect work here... this is a single site install.
(And yep, PHP Browser Detection is definitely installed and activated.)
Because it's a responsive site, there is an html5 detection script in place looking for older than IE9 so as to run html5shiv, do you think that is interfering?
<!--[if lt IE 9]><script src="https://html5shiv.googlecode.com/svn/trunk/html5.js"></script><![endif]-->
Many thanks again.