This, it turns out, is a frequently encountered problem and, having trawled through the forumz, I've found a variety of partial solutions.
So - for the benefit of anyone else who ends up treading this frustrating path, here's the solution.
If you are unable to delete your LPTENUM from the registry entry, even in safe mode, the key will need amending. Administrators, it seems, do not have permission to amend keys created during the PnP process.
So - right click, check 'permissions' and tick the 'grant full control' box.
Full details at
http://forumz.tomshardware.com/software/Delete-Registry-Key-ftopict121979.html
Thanks to all who helped me on the way to this info.