"The remaining XP machines are still in place due to complex technical requirements from a small number of externally provided highly specialised applications"
My best guess is some dismal web app that is heavily dependent on some of the non standard idiosyncrasies of IE6 (less likely would be driver issues as number of machines way too high for any hardware related driver, and for most other issues compatibility mode on Win 7 would fix teh issue)
Though I have sympathy with GMP, I do not like planned obsolescence by stopping security patches (which is what we get from all the Software vendors, be it Apple or MS on desktop or Apple and Google on mobile, and in between makers of software e.g. Firefox that only support more recent Mac OS versions) and thus ensuring a costly purchase of new hardware and software when the old system "did the job"
I run plenty of archaic low spec hardware - it just ends up having its original OS replaced by a lightweight Linux so I can choose to add key security patches