By way of update, the repairs to my A7 55TFSI in France were completed (replacement of starter generator with part no. 4N1903028 and ancilliaries) 2 weeks ago and I travelled to France to drive my car back last week (arranged by Audi Assistance). I was not able to travel sooner because of other commitments. Everything seems fine with the car.
Audi Executive Office had agreed to reimburse the repair cost if I paid first which I did. I sent off the paperwork about a week ago and the Audi Executive Office called yesterday to say payment was in hand. I received a bank transfer today via my local UK dealer.
All in all, apart from the dangers inherent in complete electrical failure on a French autoroute in heavy rain and the general frustration of it all, I think it was a reasonable outcome.
If I did not already have a service plan to cover my imminent 40,000 service, I would have had the service done in France too. The dealer labour rates seem to be about 50% of UK rates.