[Gromit]

Whilst I wouldn't dream of disputing an Audi dealer's opinion, there's one other possibility that could be worth checking before going too far down the condensation route. The PS is electric and in your part of the globe it's not only cold, it brings a whole new meaning to the word COLD, right? Check the state of your battery. It could be that if the battery is either short on charge or getting old it doesn't have the capacity to put out quite the requisite power in extremely low temperatures, bearing in mind it's just had to put out pretty much maximum power to get the car to start at a time when it's least capable of doing it.

Worth checking as it could be an early warning sign that your battery is about to let you down in a fairly big way.