Like many on this thread I had the same problem on my Superb II 4x4 TDI - taking miles and miles to warm up and then cooling down when going downhill.
My nearest main dealer diagnosed the main thermostat, but when it was changed there was little difference. Like most main dealers they charge for a consultation which proved to be a waste of time.
Thanks to this forum I explained to my usual independent garage what the problem was, gave them to part number and asked them to change the DSG thermostat.
They called up a picture of the cooling system on their database and identified where the part was and acknowledged that I was not going insane!
3/4 hrs labour later and all is now well.
Thanks everyone!