my a6 has just hit 57,000 miles on the clock when is the cambelt due to be changed ?

 

Assuming you mean the timing belt, the official Audi line on this is 100k. Most people try to get it done earlier -- on the order of 60-75k. So, you're getting close. While you're in there, it makes sense to replace other wear prone parts (water pump, tensioning pulleys, etc.). ECS Tuning has a kit that contains all the parts you need.