I hadn't tried
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8">
as I already have
<meta http-equiv="X-UA-Compatible" content="IE=IE9">
because I'm relying on HTML5 features in IE9 and I need to force it into strict IE9 mode.

Could investigate
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8, IE=9">
as you can specify more than one such tag, but I suspect you'll either get IE8 or IE9 mode overall.

Of course if you don't need strict compliance (HTML5, CSS3, etc) then this sounds like an easier fix