Going to answer my own question now.. minutes after posting. Turns out Googling found me an answer pretty quick.
The G450 sensor is the DPF pressure differential sensor and apparently it fails pretty regularly on the PD 170 engines.
Funny thing is the VW self study gives the following symptoms if it fails, which isn't what I experienced.
But aside from the different warning lights, the behaviour of the engine is reasonably explained if the pressure sensor is bust.In the event of signal failure from the exhaust gas pressure sensor, the particulate filter regeneration cycle will be based on the distance travelled or the number of hours in operation. This cycle for particulate filter regeneration, however, is not effective over a long period of time. After a predetermined number of cycles, the diesel particulate filter warning lamp will light up and the preglow control lamp will then flash in the dash panel insert. This informs the driver that the vehicle must be driven to a workshop.
I would reckon it would be pretty safe to drive but I might try and fix it in the morning. Lucky I have a VW spare parts department within easy walking distance.
BTW, this pretty much settles it. The DPF is coming out at some point, hopefully soon,