OK, uninstalled PVD then firebird, rebooted. Installed firebird in default and PVD as full install.
Tried to start PVD and got a new error about Visual C libraries. I remembered that it looked like there was something to do with these in the updated embedded client zip you posted earlier, so I re-copied the contents of the zip to the PVD directory.
Although this got rid of the Visual C error , unfortunately, non of this worked fully and we are now back at the beginning with the same error!