Heater query

[Scribe]

My car quickly comes to temperature and the needle stays just above the mid point regardless of how it's driven.  But then the ventilation blows cool air onto the screen and face (and Gentleman's Excuse Me under the steering wheel) even though it carries on blowing warm/hot air onto the feet.  Turning the temperature up to full doesn't have any effect on demisting.  I'm wondering whether there's a separate thermostat for the screen, which is faulty, and where it is?  (As the car's now 18 years old I can't really complain.)