The same pages render perfectly using IE8.
This relates to the way in which IE applies the doctype to the iFramed content; IE8 used the doctype of the iFrame content itself, whereas IE9 uses the doctype of the parent frame.
The solution / circumvention is to add an http-equiv to force IE9 to render as IE8 as a workaround.
Again, this isn't something that I've yet experienced first hand, but I'm hoping that this may help my project, where we're using various versions of IE.
As ever, thanks to James for this ...