Thank you VERY much for making the effort to give such a detailed and helpful reply. It all works well here. A pity that DXO tech support couldn't have been as helpful.
I had tried using exiftoolgui to make exif data changes, but PL4 still hadn't recognised the lens. I can now see that PL4 is using the 'lenstype' tag, which I hadn't been able to change, rather than the basic focal length and manufacturer data that I did change.
I don't think exiftoolgui makes this tag editable, which is what had stumped me. (FWIW all other gui exif data editing tools I tried didn't offer this option either)
For those that follow and don't like messing with command line programs. The most user friendly way to handle this is by creating a folder, putting exiftool into it (rename to exiftool, not exiftool(-k) ), the create a short cut to the program in the same folder, the command line can then be easily added to the short cut's target properties. Then you can just run the program on any and all files copied to the folder, then move the changed files back where you want them.
I'll probably do this to all new shoots with this lens. Going back to convert every other instance of using this lens in the past eight years won't happen quickly with over two thousand images in dozens of different folders.
I'll now go and test some more examples to see how well PL4 handles this lens. At first looks it would seem to be not any better than Lightroom, but other images might show more benefits than the first trials.
I still have big reservations about using PL4 when other lenses that PL4 should know aren't being correctly identified. Apart from the fantastic noise reduction there aren't a lot of compelling reasons to add it to my software library here, but lots of tedious little annoyances that make working with it frustrating, eg always starting with a constrained crop ratio and the rather coarse adjustment sliders, there's others too but that may just be not knowing the program well enough yet. It may be that it will be useful if just regarded as a noise reduction plug-in to Lightroom.