Rear portion of suspension sticks down then (bang!) it will jump up to correct level?
Hydraulic pump keeps activating on & off as the rear suspension starts dropping.
Have replaced a very ugly corroded rear height sensor but to no avail?

Any pointers?

Have you checked for free movement of radius arms. The anti roll bar locks the two together fairly rigidly. The taper bearings get water in them and can break up pretty badly.
Not a bad job once you get your head around it. Febi do kit with everything in for reasonable money.