Hi smurf is it already that cold there..? mine too failed last year but i've not replaced or fixed it yet.. but I can tell you the price in GBP.. if you want a new webasto unit.
I don't know if you have done a scan but it should come up with the below if it is the small glow plugs, I havent been able to get hold of the glowplugs which are only about £20.. maybe you will have more luck sourcing one?
01412 - Glow-Plug with Flame Monitoring (Q8)
27-00 - Implausible Signal
01412 - Glow-Plug with Flame Monitoring (Q8)
30-00 - Open or Short to B+
The only option was audi prices which are £280 (skipton quote last year) for the combustion chanber insert (inc glow plug).. if it is more than just the glow plugs it is £980 for a new complete unit
Sorry its not a cheap one to fix..
if you do manage to find glowplugs for the unit please update us.. im told they are used in some webasto boat heaters