Any ideas from those in the know..?

Starting my 2009 TD4_e manual yesterday in -4 degrees, a weird rattling sound could be heard on idle. Sporadic and in no regular pattern, much like an old bolt being rattled in an empty tin can. The noise disappeared when revved and has not been heard since, though he weather has been warmer. I'm wondering about the fuel-fired auxiliary heater, might that be the cause?

It may not be temperature related at all, could be something completely unrelated.

I've no clue, here's hoping the Freel2 community might have some ideas.

I think you will find that your FL2 is fitted with the normal PTC heater, which boosts the cabin warm air when first started.
The fuel fired heater is an extra.

(Positive Temperature Coefficient (PTC) Heater) FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 79k+ miles) (MY2015)
Metro in its 11th Year of (Extended) LR Warranty / Full LR Service History
(Expensive, but Trouble/Worry free - hopefully?)

throttlebody with stripped teeth sounds a bit like that?

Bingo! I've just read a helpful thread on the throttle body issue - there's a video showing the issue and it's exactly the same noise. I'll get that changed in the expectation that it'll resolve it.

Is there any merit in looking at an upgraded throttle body? Is it one of those parts that could be upgraded for better performance etc?

They have been modified I understand to be more robust. So avoid old stock. FL2 MY10 TD4 GS traded in at 2 years
FL2 MY13 TD4 GS Current

there should also show a fault code (without engine management light)mine did.