Flushing the hydraulic system
There is a product called Hydraurinçage (sometimes also called HydraFlush, the French name is pronounced as \id-ro-ra[n]-sa'zh\ or, more precisely, using the IPA phonetic symbols: ɪdrɔrɛŋ'sɑ:ʒ). It's a special cleaning liquid manufactured by Total, orange in color; your nearest Citroën dealer is sure to have it on stock. The price is more or less the same as that of the regular LHM.
To flush the system, you have to drain the old LHM, clean the filters, fill it up with Hydraurinçage, drive the car as usual for 3-5000 kms, then drain the orange fluid, clean the filters again and refill with fresh LHM.
When replacing the fluid, don't forget to drain the brake circuits as well: put the car on axle stands. On the highest suspension setting, remove the wheels. Let the engine run and ask a helper to press on the brake pedal. Let the fluid run from each brake caliper bleeding screw (put a transparent hose onto the screw) until the new fluid comes out (as already mentioned, LHM and Hydraurinçage have different colors).
There is another trick you could try first: calisthenics. Go up and down with the suspension for fifteen minutes, from the lowest position to the highest and back again (standing still, naturally). This makes the LHM circulate quite a lot inside the system (much more than during regular driving), which in many cases will free and mobilize the dirt in the system bringing it back to the reservoir where the filter, hopefully, catches everything. If the suspension or the power steering did improve after this exercise, there was dirt in the system, so check and clean the filter in the reservoir (or even replace the LHM).
Don't be alerted if the comfort of the car changes suddenly. It might become softer or harder with the flushing fluid, it might even change its behavior during the kilometers you drive. Even when you replace it again with fresh LHM, don't expect it to normalize immediately, give it another two or three weeks before you judge how effective the flushing was.
If the LHM was regularly replaced and there was no strut damage (which might have sent send metal debris into the system), flushing is usually not necessary (but if you're unsure of the earlier life of the car, it might help). On the other hand, be prepared that flushing might dislodge some dirt that used to seal otherwise leaking elements, so you might need some repairs after the flushing. Not necessarily, but don't be alarmed if some leaks surface. Even if this is the case, it's not the flushing that causes these problems, it only reveals them sooner than normal wear and tear would.