Thanks for your quick response Ken. Unsure about the pollen filter but it will be a minimum of 2-3 years since the last change (I own the car since mid 2015). Where are the drains for the scuttle plenum (sorry, I'm not very familiar with car parts...).
I have tried a few tests - leaving the car overnight in ca +15 deg C still did not get the heater blower started when idle but after driving ca 800 m and the temperature gauge rose it started. Then I had the engine heater attached and the car parked in -15 deg C and the heater blower only came on after driving about 5000 m but at a different temperature gauge reading. So it is kind of on and off, but I have 3 small kids and would really prefer to put them in a pre-warmed car where the blower works also on idle.
BTW, there's a nice webcam of Svalbard at the bottom of www.unis.no - it is still definitely winter up here in the high Arctic :-)
Any thanks in advance for your tips.