I've just got off the phone to the dealer regarding this and the burning smell when the fans are running, he said it is most definitely the DPF Regen happening and as the temperature is around 700 degrees, when you turn it off halfway through the fans go nuts. He has advised if this happens to get it out on a decent run as soon as poss and get the revs up 3000. Mine has does it quite a few times now on short round town journeys