Thanks for the prompt reply.
Without doing anything to the car, I took it for a test drive to try and gain a few more clues.
When starting from cold, the idle speed seems ok, but when starting from hot it can be anything up to 1,500rpm.
The surging is now happening all the time and I noted exactly what happens.
After selecting drive and setting off, the ASR light appears after a couple of seconds. The engine surges (revving to 5,000rpm) after I press the accelerator pedal, then immediately dies.
The revs fluctuate between 0 and 5,000rpm a few times before the engine will die completely.
Are your initial diagnoses still valid? If so, where are the components you mention? I can see the ABS/ASR module but not sure about the actuator and potentiometer.