I thought the air intake temperature sensor was pretty unimportant to be honest. Anyhow, running out of ideas I looked on alldatadiy.com where I read "The intake air temperature sensor which is located in the induction system measures incoming ambient air temperature. Information concerning the incoming air temperature is sent to the CIS-E and EZL control units and is used to calculate necessary adjustments to the air/fuel ratio and ignition timing. Low ambient air temperatures will tend to enrichen air/fuel mixtures and high ambient temperatures will tend to retard ignition timing."
So I resistance checked it, it should have the same resistance profile as the CTS. At room temperature I got 16kohm, telling the KE-ecu that the air is -30C and so enriching the mixture. This would explain my sooty black spark plugs too. I'll have to wait until wednesday for the part to come from the big merc warehouse in MK to find out though but I'm hopeful. Also would explain why without the electronic side of things the car performs better, as the base mixture is good.