Here's the new vents, about £15 on eBay. The seller had left the hazard light switch in the housing, so now I have a spare!
I had previously asked the seller to make sure all the fins moved when the controls were used, and checked myself before installing the new set of vents in the car. Unfortunately, one of the fins on the passenger side vent was broken, arrowed below.
Here's a close up of the problem. Each fin has a central post to rotate on (circled in red), and a post that the control wheel swivels up and down (circled in green). The bottom fin's swivelling post has broken off. It is hard to believe these tiny plastic parts were considered suitable for withstanding everyday use in a car. These vents were very clean and obviously hardly used.
So I had the old centre vent unit with a bad right hand vent, and a new centre vent unit with a bad left hand vent. I cannibalised the new unit to repair the old one by prising out the working vent.
After reattaching the hazard light cable, slotting the fixed centre vents into the dashboard:
...Thanks for reading.