240 General: Ailing Nirvomats?

[Clifford Pope]

I'm pretty certain my car must have Nirvomats at the back, or something similar. The kind that have an external spring for part of their length?

I noticed today when fetching 5 heavy bags of compost that the back end seemed rather low. Even after driving for a bit it had only pumped itself up a bit, and still looked sagged. I'm sure in the past it would indeed self-level virtually to normal ride height.
When I unloaded the bags and rocked the suspension a bit the level rose by about 1 1/2 ".

Presumably this indicates the shock absorbers are ailing? They still pass the bounce test, and the recent MOT, but don't seem to self-level any more.
Also they have developed a squeak when standing still.

I'm aware that Nirvomats go with special softer springs, so can't just be changed for ordinary shock absorbers, but are they really worthwhile? I've several spare ordinary springs so wondered about using those and just getting standard (much cheaper?) shocks?