i doubt youl get any heat with the tdi just idling, they run very cool i find , if i keep the revs below 2000rpm for my 15mile journey my temp gauge will not go above 60 degrees although ile have heat pouring from the cabin heater after a few miles , the engine will not get hot unless it is pushed above 2000rpm and then it will rise to the normal 90 degrees running temp and its the same with the webasto running too , they just dont make heat unless pushed a little ...
As you say its minus 10 id say it must take forever to get hot and certainly wont make the temp gauge move just on tickover
first thing i would check is all the fuses and earth points as can cause issues , then id be checking the dosing pump conections , underneath the car under the rear seat location , deisel cap side of the car , youl find a silver coloured alloy heat sheild underneath the A2, behind this youl see a small round pump with fuel hoses conected to it , remove and clean up the electrical plug, if you dont have access to vag com id even go as far as rigging up some wires with a 10amp inline fuse and test the pump works when connected directly to the battery, if the pump works ok id then go to the webasto unit under the bonnet, youl need to losen all the 10mm mounting bolts to be able to move the webasto out a little to gain access to the 2 wiring plugs , id sugest cleaning these aswell with electrical cleaner..
but without vagcom youl be hard pushed to find a fault without any fault codes stored in the webasto controller