My Volvo has started to get a bit of a lean on to one side, at the rear the car sits lower on the right than on the left.
I can fit a finger or two between the arch and the tyre top on the left, there is no gap on the right.

Is this likely to be because the leaf spring has gone soft or could my dampers be damaged?

Leaf springs are original but the dampers are only 4,000 miles old.

My physics says that dampers don't take any of the weight except when they are being squished in or out so it will be the springs - sounds just like my 1.7. Ta bog.

Do both dampers move OK? It should not be shocks but never heard of saggy rear springs before, are the mounting points OK? Have you driving it hard?

I had this problem, it seemed to go away after I let the preload off the torque arm, when I fitted lowering blocks.
May be worth a try loosening the nut on the torque arm, jacking up the axle, letting it down and then tightening the nut again?

Also check the rear leaf spring hangers as if they rust out the shackles will push up

are you using landy shocks ?