Hi... with the help of your tech support people (which took us ages!), this is now solved.
For information (in case anyone else has this problem in the future); the CUPS filter was correctly installed, the problem was actually caused by the placement of the PPDs.
When I used the Quark PPD manager (using the 'Auxiliary Folder' > Browse), we couldn't actually locate the Xerox Phaser ppds, or any of the other ppds. They had actually been physically moved somehow (I hadn't moved them), out of the 'en.lproj' folder, where Quark was looking, back one level into the Resources folder (Macintosh HD > Library > Printers > PPDs > Contents > Resources > [en.lproj] ).
Once I browsed to the Resources folder and updated the list of PPDs using the Auxiliary folder > browse, I was able to 'unclick' the PPDs I didn't want (tedious job, but only done once) and everything now works fine.
Thanks