SPARE

[MikeFish]

Sorry for the delay, just got back to the forum after the weekend.
The spacesaver is: T145/60 R20.
I got mine off eBay from a breaker - it is the same spacesaver as is fitted to many A6 cars, so easily available. It was listed as "never used" and, indeed had not been. I bought mine in 2016 for £140. At the time, I religiously checked the rolling diameter to ensure it matched my 285/21" wheels accurately enough to not be an issue. I think RS6s come with this exact space saver in some countries.
It fitted straight into the boot.
I also bought a storage bag to put the full-size wheel into should I have to put it in the boot. That is on eBay described as "SPARE WHEEL TYRE STORAGE CARRY BAG COVER 14''-20'' 285mm width SINGLE" for £9.
I can't quite remember, but I think I needed to top up the toolkit with a couple of tools to assist easy changing (wheel wedges, better bolt wrench...?).

Did it fit over the front calipers or rear only?

[wildbore]

Sorry for the delay - just seen this question. The space saver fits over the front brakes no problem (first thing I tested when I got it) - I have standard steel brakes; can't vouch for the ceramics.

[morski]

285/30/21 has a rolling diameter of 704.4mm whereas a 145/60/20 is 682mm. Thats a 22mm difference in height and a 3.3% speedo error. More so wouldn't this cause an issue with the diff/ABS systems when it calculates incorrect rolling rad/dia?

[wildbore]

It's a fair point, but I thought 3% different in rolling diameter was supposed to be the limit of what is acceptable for the diff (I think it is only the centre diff that is susceptible to rolling circumference differences, and if you only have one space saver on an axle, that halves to 1.5% at the centre diff, anyway). And it is an emergency, low-usage item, I am not going to drive for 1000s miles on it.

I remember that I did calculate that a 165/60/20 tyre is an almost spot-on match for the 21" wheels, but it won't fit in the wheel well in the boot, so the 145 is a better solution in my mind.