Welcome to the forum Wayne, you'll find the members on here are a friendly and helpful bunch :)
Realistically you'd be looking at mpg in the 40s or 50s on a mixture of motorway, country roads and town driving. But if it's just town driving then this may drop to mid to high 30s.
One thing to check is whether or not the a6 has had the emissions recall carried out and if so how long ago. If it's been done in the last month and now the car is for sale then you have to question why. Have a look on this thread about the recall and the issues that can appear afterwards:
Also here is a buyer's guide on the c7:
Hope I've helped :)
Cheers
Steve