Thanks again Kyle and abc. Just a quick update - I started a trial of Adobe Acrobat Pro and played around with changing the colourspace in a PDF exported from Word. After much head scratching and research, I eventually managed to change the colourspace within the 'Print Production' options.
I verified the conversion had applied to all objects by inspecting them with the Object Inspector part of Output Preview. The ColourSpace of everything had changed to 'DeviceCMYK', which I assume is correct.
Reassuringly all black text had been converted to a CMYK of [0.75,0.68,0.67,0.902], which I assume is an acceptable representation of Rich Black. Please correct me if I am wrong.