A quick follow up to this one.
I finally followed the advice on removing the glovebox a week ago, and quickly spotted the control arm for the fresh air. Much to my surprise, it was working ok, a bit hesitant, but definitely moving the full amount, and nothing disconnected.
So I pulled the scuttle panel, only to find the cabin air filter was so blocked it was, for practical purposes, waterproof.
WTF!? This car has been serviced absolutely religiously since it was purchased.
So I got onto the mechanic who also went "WTF?! That model has a cabin air filter?!" Turns out he didn't know the filter was there, so it probably original, or close to it. 22 years old. Pulled it out and hey-presto, perfect air flow.
For a week.
Now the cabin fan has died, but I will start a separate thread on that issue.