All sorted. I removed the slats from the spare vent and installed them in the existing vent.
No point wasting time trying to swap the old vent out as the "new' one had a broken vertical slat.
You don't need to bend the horizontal slats, or pliers, just use a "pick" tool.
See photo, the set cost £3 from eBay.
The whole job took less than an hour (it's a bit fiddly).
I warmed up the replacement vent parts as I was concerned they might be brittle if cold..
The photos show (1) the "spare" vent, with the broken vertical slat, (2) the "pick tool", and (3) the "new" slats in place.
I forgot to take a photo to show the original vent with the missing horizontal slats, and slider.
Many thanks to everyone here for their advice and help in pointing me in the right direction. (Esp
@Andrew)
Next up: replacing the broken cup holder..