There is a Citroën dealer here in The Netherlands who can flush the automatic gearboxes from more recent Citroëns (Xm, C5, C6) They do use the cooler to connect the flushing machine to the gearbox. To do this they remove the 2 bolts that secure the cooller to the engine and replace them with 2 specially made plugs that connect to the flushing machine. The gearbox pumps the old oil out and new oil is being fed to the box (same amount out and in). They continue until the oil that comes out is clean. After that they will leave it running for a while with the new oil being led through a filter to catch any dirt that absorbed by the new oil. They do this as a service for a reasonable price (around 300-350 euro's if I recall correctly).
After this your box will have no old oil left, but only fresh oil. The only thing that they obviously can't clean is this way is the filter of the 4HP20 box as this requires splitting the box. They use the official method to make sure the oil level is correct once the cleaning cycle has completed.
I've had this done to my box, but my results weren't as good as others have had. Main reason for this is that my car already did well over 200.000 kilometers and most likely the gearbox oil was never changed in this period. Their advise is to flush every 60 or 80K kilometers or so to keep the oil clean.
My box failed 10 to 15 K later anyway, but it would probably have failed earlier if I wouldn't have had it flushed. People with newer gearboxes have great results with the box being very smooth again and behaving like new.
I'm at the point where I'll try to overhaul my gearbox my self and probably will put this flush in my "regular maintenance schedule" afterwards. I'm not sure if they do this in the UK yet as there do not seem to be many who can perform this service....
edit: The garage is:
http://www.oldenhage.com/garage-johan-o ... k-spoelen/" onclick="window.open(this.href);return false;
and much can be read about this in the following topic on the Dutch C5 forum:
http://www.c5club.nl/c5forum/viewtopic.php?p=373857" onclick="window.open(this.href);return false;
