Air conditioning on full cold

Audi A4 2.0 TDI (143) Executive S Line S/S 2010

My Air Con (3-zone) does not work as expected, I assume it is thermostatically controlled but it doesn’t seem to cut in and out at the required cabin temperature setting. It is just on cold all the time.
I have to turn it off and on manually to control the temp. If I turn the temp up to above the outside temp the fans stop but I still get cold air coming through the vents.

I only noticed this on a long motorway run in the real hot weather.

Are there any checks I can carry out? Is something broken?

Thinking back to winter time, I thought I had cold air from the central vents all the time even when it was supposed to be heating the cabin, so I turned these vents off and rely on other vents to heat.

Any thoughts?

There is a setting in MMI under` AC`, for adjusting the output temperature of the center vents only, to cool or warm.
This may be your issue.