Hi Mark, I also have an A4 Allroad 2017 and had pretty much exacty the same problem - car asked for adblue, filled it up and message turned to adblue fault 650 miles no start. It has in the past taken a day or two to go out, but now driven 400 miles and still says 650 no start. Taken to an Audi speacialist (not a dealer as two in London told me to take a hike for a fews weeks and then they could fit me in to do a diagnostic). Faults are Lost communication with reductant control module. and Reductant pressure to low. To fix I need a new Adblue controller £690+VAT and a new pump / tank £941 + VAT total with labour £2,665, so not quite as horrendous, but using a specialist not a main dealer. Having discussed with some Audi mechanics, this seems to be a very common problem which Audi are not being as open about as they might be....