As long as you steer clear of OEM new parts then the struts are not that expensive (compared to new).

For example.
https://aerosus.co.uk/audi/a8-d3.html

You only need to replace the leaking side, I dont think recalibration is needed either unless you disturb the level sensor.

Thank you for the link. My head tells me replace both sides as they are the same age and have had the same upbringing, is that not the case? (£ aside!).
After 35k wonderful miles this appears to be my first proper expense, so not bad overall!

I myself got a loud noise on warm start-up for the first time yesterday. I have also suspected the compressor coming in more than before in recent days. I read the codes in the picture today. So I too will need to explore what's going on I think. (If indeed the noise I heard is related):