Problem solved!!
I went over to MIL's again today and tried again to install the X85 driver from the Lexmark site. As a precaution I used another of the USB ports and the installtion proceeded exactly as before to the point where I was asked to connect the printer's USB cable. After that, the installation proceeded as before and seemed to hang, with no more progress/instruction/dialogue boxes displayed on screen at all. The hour glass was flickering on/off peridodically but there seemed to be no progress whatsoever
As previously I left it ticking away for about 15 mins and then, fearing the thing had stalled once again, I closed the Lexmark download site and a 'setup' box suddenly appeared in the task bar. I clicked on that and the remaining sequence of installation/setup boxes appeared on the desktop as normal and the process was completed successfully.
I still don't know (or care) why the original printer which is still installed is still showing as offline when it shouldn't be but think the X85 installation problem was down to a failure to display some of the final setup dialogue boxes on the sreen and hence we were unable to select the final options and complete the process. Why this happened on MIL's PC and not mine when I did exactly the same X85 installation on my PC a few weeks ago defeats me but who cares! If it works don't fix it I say....
Thanks for the help everyone!