Quick update. The rear springs and shocks where replaced with Bilstein B3 and B4's, it's only a 320d driven by the wife so doesn't need any trick suspension.
The springs were undamaged, and compared in height agains the replacements, were slightly 5-10mm longer. The shocks that came off were knackered, easily compressed and probably still compressed now as you had to pull the shaft back out of the shock.
With the new springs and shocks fitted it is back to even height between the wheel and arch. The suspension performance is noticeably better, feeling firmer and better damped even though the new ones are only supposed to be OEM replacements not improvements.
It was suggested that if a shock wasn't working it allowed the spring to be more active, which would eventually lead to it sagging before failing. I can only assume this was the reason for only one corner been lower than the others.
Failed gas shock was probably causing the sag, oil type shocks don't return once compressed gas ones do, have a pair of bilstein group n shocks in the garage takes quite a bit of my body weight to compress them