Was talking to the dealer the other day and asked about the C8, he said they had just been over to Germany and there was no mention about the new A6, and said it was unlikely to be next year. So looking at autoexpress and a few other sites it does look like the next model will be released in 2018.
http://www.autoexpress.co.uk/audi/a6/95640/new-audi-a6-to-arrive-in-2018-with-sleek-new-look
http://www.autoexpress.co.uk/audi/a6/95640/new-audi-a6-to-arrive-in-2018-with-sleek-new-look