[Adrian E]

Is the seller the same bloke who bought it from the Audi dealer where it was traded in, or has it gone through a private owner since?

I'm not sure how easy it is to check, but I'd hazard a guess that a 10 year old cambelt isn't going to be in the first flush of youth and will show heavy signs of wear inside the teeth. How much work is involved to remove a cover to check I'm not entirely sure, as I'd never entertain doing one myself. It can be tough to tell a 2 year old belt from a 10 year old. If it's not marked up as an Audi part or the OE equivalent it at least suggests it's not original.

Can't imagine the cost will vary much between 32v and 40v so whatever you paid out on yours in the past will probably be accurate enough. It's a big bill from Audi but most independents can do it for more sensible cost. As an idea my A6 V6 cost £600 of which £450 was Audi parts! Labour would normally be a much bigger bill....

With no written proof I guess it's best to assume it's not been done until it can be confirmed it was by visual inspection? Even then it doesn't sound like there's any evidence of mileage or time since it was done to know when it'll need doing again!