I recommend owners of older C5s occasionally check the temperature of their front wheels after a drive. A warm (or very hot!) wheel will indicate a binding brake which is not otherwise apparent - until the pads catch fire!.
A few months ago, when I stopped, the front right brakes were smoking, almost on fire. There were no indications whilst driving, and using cruise control may have disguised any sluggishness. The pads actually looked ok, so I removed the caliper, generally de-rusted the pad support and re-lubed the guide pins. For the following weeks I only drove short journeys and checked for a hot wheel after every one. Sometimes the right wheel was warmer than the left wheel and other times the same - so I had an intermittent problem. Indeed, I now think that this problem may have been occurring for a long time, and may have caused the previous pads to partially delaminate through unnoticeable regular over-heating.
So, the next focus was a possible sticking piston, and I note Coastline Taxis advice on sticking handbrake lever arm on the caliper. However, I decided not to mess around anymore and fitted a new caliper. A reconditioned Bosch caliper from Mister Auto took over a week to arrive but at a cost of around £75-80 (after refund of deposit on return of old caliper) it was a quick and best fix. I am still monitoring the wheel temperature just in case the new caliper has merely disguised another cause.
I dismantled the caliper before returning it to Mister Auto. It could have been sticking; there was no rust on the piston but the walls had numerous black deposits which rubbed off with solvent, presumably rubber from the fluid seal. The seal itself was not leaking. If the left caliper were to exhibit the same binding in the future I might just renew the fluid seal first before replacing the caliper. Just a note on removing the piston if the caliper has been disconnected from the brake hose. Simply operating the handbrake lever numerous times will extend the piston most of the way (watch out for fluid squirting from the hose hole). When it stops extending, it means the piston has reached the end of its threaded worm-screw and is now free to be removed. Then its just a question of pulling out the piston whilst twisting. Make sure the dust seal remains stuck to the caliper body.
Another factor on the 2.2Hdi is the air deflectors which are prone to rust, fatigue and failure. Both mine departed several years ago. Now, the metal brackets are no longer available and I have yet to find another C5 with them still intact.