HI Gareth,
Thanks for your advice. I've checked the distance from the wheel arch to the center of the wheel and it's 34cm on the offside and 37cm on the near.
To be honest, I'm not convinced it was ever right. As soon as it was done I didn't think it was completely level but assumed it was down to my driveway not being level. I've since checked it on a completely flat surface to confirm it's definitely off.
By 'third-party spring' they basically mean it isn't an official Audi part (which I assume would be significantly more expensive). They didn't mention this at the time but being honest, I assumed it wouldn't be given the price they quoted. I would still expect it to be compatible though. I even stressed to them that it is the standard A6 Avant and not the S Line (which I know will have different suspension) to make sure they fit the right part.
It's possible that the new spring is broken, and I hope it is as that will be a more straightforward conversation. However, it definitely isn't sagging as much as it was when it originally broke so I'm expecting that it's fine from that point of view.
If I allow them to fit the same spring to the other side, then essentially the front of the car will be 3cm lower than it was originally. This sounds like a big drop to me, which may effect the handling, but I'm not clued-up on this kind of thing to be honest, so if you have any advice then great! I'm hoping they can just fit the correct spring, even if it's more expensive and I have to pay the difference.
It's frustrating because I only bought the car (used, from Audi) in October last year and it came with a 2 year Audi warranty. The fault happened right at the start of lockdown and Audi promised to call me to arrange "inspection" as soon as they were open. I waited as long as I could before paying for it to be fixed (I wasn't confident the Audi warranty would cover it anyway). They still haven't called me to this day, but, assuming the spring isn't actually broken again, I'll now be unable to take it to them anyway!
Thanks again for your advice. Much appreciated