I stumbled across a piece of software called PrintFab 2 which is, according to the maker's blurb, a
"printer driver suite with RIP functionality. PrintFab's innovative “dynamic” color profiles ensure full control over color mixture and ink consumption combined with perfect print quality."
Installing PrintFab 2 sets up a "new" printer according to the configuration info you supply and loads the PrintFab driver as it's default. It's a simple enough process, and it restores duplex functionality to my DeskJet 970Cxi. It does an awful lot of other stuff by the looks of things but I'll investigate that during the next 30 days of my free trial.
PrintFab 2 may be an overkill solution, and it isn't cheap, running in at about £45.00 for a single user license for the simplest home version. But for peace of mind, saving trees, and saving the time I waste looking for alternative solutions, I think I may have finally found my Holy Grail, and I don't mind paying a small price for that.