UPDATE: I was right! I am more knowledgeable about the IHKA system than their own master tech!
The dealer insisted that they try to fit the part their master tech confirmed didn't exist, as that was the only way they could be 100% sure that I was right. After having it for a few hrs, they called, apologized for wasting my time, and confirmed that a UK spec E92 335i does not have a heater control valve.
They are now re-investigating for free and will be inspecting the actual heater unit to check flap movement. Result - that woulda cost me loads if I was paying!