Self-levelling dampers?

[nOO7]

At the major service yesterday at an independent VW specialist it was noted that my NS rear damper was leaking. Apparently the car (B6 2.0TDi 140 4Motion Sport Estate) is fitted with self-levelling dampers. Has anyone else got/heard of these? The part is dealer only and currently on back-order and they weren't able to give a delivery estimate. A quick look around online brings up Nivomat shocks, which appear to be several times more expensive than standard ones - great .
Has anyone else got them and had them replaced? Any idea of cost?

Thanks