On most other cars I would be pointing you in the direction of the dampers (shox). However the Silver Spur is not fitted with conventional dampers. Instead it relies on 'gas springs'(also called spheres) which perform the same function along with a self-levelling facility to keep the rear of the car at the same ride height irrespective of the load being carried. These springs are part of the overall hydraulic system which is fairly complex.
First you should check that the level of mineral hydraulic fluid has not dropped below the minimum mark in the fluid reservoirs and top up if necessary. Failing that I would be looking at the static gas pressure in the precharged spheres which should be in the order of 1,000 psi (68BAR). If it is significantly lower than this figure the spheres will have to be changed for new ones as the official factory line is that they cannot be recharged. New spheres can be obtained from the usual suspects for around US$100 +S&H in the UK.