Mk3 1.6CL Cold Start Problems

Golf 1.6CL. Runs perfect when warmed up, but will only run from cold when holding the revs on the accelerator. A new coolant temperature sensor has been fitted, and the throttle body has been cleaned and reset. Butterfly is not opening slightly on idling when engine is cold. Looks like either ECU is not sending message to the throttle position sensor, or the sensor is not doing what it should when a message is sent.

Anyone any thoughts on most likely problem - sensor or ECU? Can sensor be removed from throttle housing and checked/adjusted? Can ECU be checked/tested?