Final update on this - enough time has passed now for me to declare the alarm ultrasonic sensor repair a success.
It's gone a full week without any false triggering of the ultrasonics, when previously it would go off without fail in less than 30 minutes.
So to conclude I'll provide a summary of the fault and the repair. In my case the fault was the transmit transducer on the left hand door - probably either a dry joint internally or a crack in the pizeo-electric crystal. Diagnosing and confirming the cause of the fault is made easier with a small oscilloscope or a Lexia, but neither are actually required to fix this problem.
The first step is to confirm the ultrasonics as the cause of the alarm going off - if you have access to a Lexia you can read the 15 most recent alarm activations (and also clear them) so if nearly all of them are the ultrasonics its a sure bet that they are to blame. If you don't have a Lexia, simply hold down the alarm button on the dashboard until the alarm light comes on before exiting the car and arming the alarm - this disables the ultrasonic sensors (one time only) but leaves the alarm and door sensors active.
If you can do this for a week with no false alarms (and you normally get them more frequently than this) then its a reasonable bet that this is the problem. If remembering to hold down the button before exiting the car every time is too inconvenient, the alternative is to unplug the yellow plug to the left hand door ultrasonic sensor, which will permanently disable them.
If you have a portable oscilloscope (who doesn't
) try measuring the drive signal to the transmitter on the left hand door while its still plugged in - if you see the amplitude varying in any way, but the amplitude is steady when the transmitter is unplugged, immediately suspect the transmitter being faulty. Likewise you can measure the output of the receiver on the right hand door (it doesn't need to be plugged into the door) and if there is any rapid variation in amplitude the receiver is faulty. (If the transmitter was ok)
Note: for this to be valid the windows and doors must be closed and you must be outside the car, as the slightest movement inside the car - even your chest moving with breathing, will cause the amplitude to vary.
Now that you know the ultrasonic sensors are to blame, what to do about it ? If you just want a quick fix so that that damn alarm stops going off in the middle of the night, you can lock the car with the fob, and still have some measure of protection thanks to the door sensors, simply unplug the yellow plug to the sensor in the LEFT hand door. This will permanently disable the ultrasonics.
If you want to fix the problem properly, look further back in this thread for the full details. The ultrasonic transducers are relatively cheap - £7.95 each (compared to about £40 and £90 from citroen for each) available from
http://www.mansky.co.uk" onclick="window.open(this.href);return false; - they did charge me £15 for shipping, but that was due to them sending them by next day signed for delivery due to a miscommunication during ordering. (They can only take orders by phone or email and credit card details only by phone) The part numbers are 40LR12 (receiver, right hand door) and 40LT12. (transmitter, left hand door)
It's not necessary to fully remove the door cards to remove the sensors, heres how:
Remove the small torx screw beneath the electric window switches and pop the plastic plate with the switches out. Remove the two large torx bolts behind this plastic plate. Remove the two large torx screws either side of the flip open map pocket, (can only be seen with the top bit flipped half way out) use a flat blade screwdriver to pop the cover off the speaker, remove the two torx screws between the edge of the door and the speaker, do NOT remove the screws from the speaker itself, it's not necessary. Nor is it necessary to remove that infernal lock button that is a pain to get back on!
You can now pull carefully but firmly on the door card down the side nearest the door hinge and it should let go. Sometimes a bit of persuasion with a long screwdriver is needed to lever it up. This bit can be nerve wracking because it will sound like something has broken when it lets go, and it is actually quite easy to break those plastic clips, but somehow in removing each door card multiple times I was lucky and had no casualties. (Not so on other Xantia's...
)
Once the clips let go you just need to lift the card away 2-3 inches to give enough room to unplug the connector and then prise up the triangle piece of plastic that holds the sensor. It's just held in by three plastic pegs that push into 3 tight nylon sleeves in the door, just lift the edges a bit with a screwdriver then just pull it evenly out with your fingers.
After refitting the new sensors, it comes time to test they're working. If you have a Lexia this is easy - go to the live sensor data (where it shows the state of all the doors being open/closed etc) and sit dead still in the car with windows and doors closed. Raise your arm and move it around a bit and you should see a response from the "volumetric" sensor. It should be quite sensitive - any moderately fast movement within line of sight to the sensors should trigger it.
If you don't have a Lexia you'll just have to try tripping the alarm. Sit in the car with windows and doors closed, lock the doors with the fob, sit perfectly still for at least a minute (the ultrasonic sensors don't activate for 45 seconds) then move your arm - the alarm should go off immediately until you unlock the doors with the fob. (Now observe the puzzled looks of your neighbours peering through their windows as you set off the alarm whilst sitting inside your car
)
Assuming that test passes we know the sensors will respond to a break in, now all that remains is to see that there are no false alarms - just use the alarm normally for a week or so, if there are no false alarms, job done.
, reply to a very useful post and one I am about to make use of. Only problem is that the pics - worth several thousand words - have expired due to the lengthy time lapse. Any chance of pm with pics, or perhaps much better, making this a sticky complete with pics in the txt???? - if it can be done, of course. 
