Having tried all the options - such as moving the lever etc. - (we bought a new link rod) - and yet still couldn't solve the problem I took the auto into Mercedes Benz UK for diagnostics and the report was -
SHOCK ABSORBER FAILURE. So, the system needed depressurising, the shocks changed and then re-pressurising before finally re-setting the ride height.
That's the end of the long hike!!