Heating issue 2015 A6 S Line ultra

[Randomcat]

Does anybody have any idea why I have no heat coming through the vents. Last year it stopped working on the drivers side, now it’s gone in both sides. I’ve had it flushed and checked on a diagnostic machine at various different garages and it shows no faults. I’ve been told it’s the heater matrix. It’s still got hot water going through it and the silica bag is still intact. It just blows cold air or Luke warm air through. 
mad winter approaches it is going to become my difficult to drive. I travel a lot so I need my car and love my car. I don’t really want to have the dashboard taken apart either. 

[Steve Q]

If various independent garages haven't found the fault then it might be worth taking it to Audi.