As for servicing 20k is far too long even with fully synthetic oil! Our C5 is currently on a 6k service interval and every 3rd service has the gearbox fluid partially changed(you can only do about 3litres at once easily) and it had its cambelt, tensioners, water pump, coolant, aux belt & tensioner changed at 60k/5years. Again we think these intervals are far too long and only there as by the time it's reached the car is out of warranty and means it's no real risk to citroen for a warranty claim.
Oh and the best specialist in Cheshire imo is in stockport, Paris Autos. The owner has a C5 exclusive himself so knows them well
