Hi,
Thought I might chip in
I use my Van at worst every 2 weeks.
On the idle to warm or not I think the best way to warm an engine is to drive it, but diesels with turbo you need to be careful not to put the engine under too much strain until warmed up (like any engine actually)... the ignition of the diesel under compression in the chambers gets more efficient as it gets warmer, the oil is thicker and lubes less well (think of the turbo bearings), metal parts haven't dilated to there "operating size"...
So drive softly, and best indicator would be oil temp before you load the engine...
On the other hand (as we have here) in snowy winter conditions I often start the Van and set the heaters on full and then go about clearing the wind shield etc...
So in conclusion it is 6 of one and 1/2 a dozen of the other