not sure if you have had this done as yet but would be good to know. My A6 has done almost 60K and I had the same recomendation from my Audi garage when they recently serviced (£299 incl all filters etc.). Cost though is £429 as its part of their fixed priced offering (they have other fixed price offers for any car over 3 yrs old). The handbook states 80K as you say with no time period although Audi now seem to promote 5yrs time line guidance.

I have no connection with Audi but while they offer competitive servicing and repairs i'll go for that stamp in the service book plus any audi updates any day - by the way they did an audi recomended fix at no cost to a hose that apparently can leak due to boot opening a and closing wear and tear. If it leaks the water kills all kit hidden in boot area which is expensive!

I thought the interval was now 60k or 4 years whichever is sooner.

As well as a cam belt kit (including tensioners) it's usually best to get the waterpump/antifreeze changed at the same time whilst it is all in bits.