Never thought I would have to ask this question but the temperature hit -22 in France last week!

What is the lowest operating temperature of a TDV8?

The pre-heater heats to 85C I think, so I would guess that the engine could comfortably run down to -45 (would still be starting very easily at 40C as long as its been pre-heated) 

The lowest TDV8 operating temp will be solely governend by the quality of the diesel fuel (the amount of anti-caking agent therein) and the state of your fuel filter.

As it gets significantly colder, the diesel will not freeze but it will begin to take on a waxyness as the hydrocarbon chains begin to coagulate. The first time this will be noticed is when the wax clogs-up your fuel filter and prevents the diesel from entering the engine.

In the old days, it was not uncommon for diesel powered vehicles to 'freeze-up' at anything between -5 to -15 deg C. However, modern diesel fuel will run freely well below this level. Furthermore, the amount of water in your fuel system will have an effect on the running capability of your engine at low temps.

I reckon you would be looking at temperatures below -30 before you start to experience problems with a modern diesel but this figure would be reduced (increased?) depending upon the quality of the pumped fuel and the condition/water content of your fuel filter.  The older I get, the more I realise that people confuse wrinkles for wisdom