Many thanks for that excellent info Dave.
I have replaced the alternator and then had another look at the control wire waveform. It didn't look any different except for a slightly higher amplitude due to system voltage being higher, but the duty cycle went from about 15% to just over 30% (fluctuating to 37% under load).
The vehicle was towed in for the battery/alternator problem so I will let the customer know that that is fixed and ask what he wants to do about the P0014. I suspect he may want to book it in for a more convenient time.
As footnote for future searches, these have a one way clutch decoupler on the alternator and I have previously had VAG vehicles fitted with these that have lost the drive. This was the first thing checked on this one but it is very easy to overlook.
Thanks again.
Bryan.