From my understanding it's the control unit.
My Cruise Control and Stop/Start function won't work consistently. Rarely works in fact. I have an OBDeleven, and the only faults are the voltage stabiliser and an Engine code:
U111200 - Function limitation due to implausible message
I brought the car in to a dealership a couple of months ago for service and a new battery. Paid extra for diagnostics to be done. They said that they fixed my Cruise Control & Stop/Start issues. They didn't. They wanted to do a few repairs which would have cost about £4500, which I got done locally for about £700. So I'm reluctant to go back.
They told me that my voltage stabiliser must have gotten water damaged in the boot. But never quoted me a price either which is never a good omen.
Mechanically the car runs fine. I just miss the cruise control. Thanks for the help Jason.