A Luta Continua - My Merc E class

Yes. It's nothing to do with air suspension or air springs. (And the E has separate levellers for its xenon headlights; you can see them do a test waggle when the lights come on.)

My ignorance, in part. The full adaptive air suspension option is called Airmatic and uses air springs on all four wheels, but is confined to the top models like the E500, not for the likes of us. But all S211s (estates, that is) have pneumatic (not hydraulic, as I thought) self-levellers at the back, with a single, central level sensor. There seems to be an engine-driven compressor that feeds both reservoirs, but the reservoirs themselves can develop holes or cracks, presumably from ageing. Could be what you're observing, maybe exacerbated by some unintentional rough treatment.

I see. Thanks. If you wanted to buy one of the items in bold above as a spare part, what would you search for on the internet?  What does one look like and where is it on the car?

Try some washing up liquid and water and spray it on everything that carries the air and look for leaks when it's up and running.

One side (rear only) always lower by 2cm.

I don't think it'll be an airbag leak then. I've not had this on a Merc, but my BMW 5 Series estate had similar.

Each side has a sensor I think. My guess is one of those is bent / moved / or something.