Numptie of the highest order
Re: 20K Service
I think you'll find its now illegal for car manufacturers to invalidate the warranty as long as the servicing is carried out using the correct parts and schedule etc.
here's a quote from AmD's website too which is how I understand the law.
"Little know to most, is the new Block Exemption Laws allow us to service your car, using genuine parts the Manufacturers Warranty is still valid. "
so it's up to you what you do, although I personally would probably stick with Audi until the warranty is up.