Your service centre is not fixing the issues. The heating will blow hot and cold if sorted. In-between settings and large and quick temp changes upset it.
Firstly, check the 2 expansion tanks for any signs of water leaking, if they are they need changing under warranty to get the system to work with the In-between temps. Check they are both topped up with coolant. Try doing the bleeding procedure that is available on here. Do this a number of times, after long runs and engine has cooled down. Make sure the refrigerant is topped up by dealer. If all this is done your heating will work. You may need to turn the aircon off at time s to get the mid temps when in auto or manual, sometimes you won't.
If you really want to sell, and i believe your selling issue is due to location from a dealer, i would contact Chester as they tell me they can't get enough second hand ones. But please try fixing first, saves loosing a lot of money and you will start to love it
The heating system needs to be treated gently when working, just one temp click change at a time and wait. If it insists on blowing cold on mid temp after a long spell of full heat, try a blast of full aircon cold and then turn aircon off for mid heat and use in manual.
Ultimately, the heating system should work, your dealer is just not doing their job
