Found it ! 

 

Only had a few seconds to video the smoke which appears to be coming from the DPF to DPF sensor pipe - hopefully just a leaky pipe. 

 

Edited 13 October, 20214 yr by b1ackb1rd

I have never noticed smoke as my regens mostly occured while travelling,

the smell and slight roughness to the engine worried me the first time

they happened and I think the engine ran on when parked. It does get hot

though!

It may not be related, but my grandson had a Differential Pressure Sensor overheat and partially melt on an Audi 1.6 TDI. It was replaced and no further problems have occured. If you do need to replace the sensor it should be noted that an 'adaption' needs to be performed (with VCDS or similar) before starting the engine.

I believe part number 3C0131552R

The video was taken during a regen, the smoke appears to be coming from the short 8", open,  'atmospheric' pipe of the DPF differential pressure sensor.

 

Should smoke be coming from this pipe?

That sounds like your pressure sensor is goosed.

 

There should be a membrane between the two sides of the sensor that stops exhaust gas exiting on the atmospheric side.

 

Seems like that is broken allowing the exhaust gas to escape & will have cooked the sensor.

@PipH Thanks, I didn't realise that