David
I have EXACTLY the problem you describe on a German made van "designed for the British market". I understand we may not ID the make/model here for legal reasons but if yours begins with a "G" I'd like to know (mine does) because the OEM denies there's a widescale problem and I don't believe him anymore.
If so you will need to do everything that follows here to make it anywhere near acceptable, all of which (but only after considerable pressure) I have managed to have done under warranty at no cost:-
1. Fit remote sensor as described above (ABSOLUTELY ESSENTIAL).
2. Shorten, by re-routeing underfloor ducting, under and across front door (need to go in and out of chassis via existing apertures)
3. Properly insulate the run in 2 above. My workshop didnt do this on first attempt and it was useless for forward area as a result.
4. Cap off internal duct immediately after second butterfly valve in both front and rear.
5. Tape over any "bleed" perforations in retained sections
6. Adjust bias valve on rear of heater to give max flow to forward area.
One last request please. ***** like hell to the OEM and your dealer and post your findings on the Owner's website as individuals are being made to think they have an isolated problem. I now know of at least 4 others who have this problem and the van cannot be used on electric during cold months whereas companions' units are fine.