Original problem resolved
This may help others.
Automatic gearbox enters emergency mode (3rd gear only). Stopping and restarting the engine restores normality but temporarily. At worst this could be for 150 yards of driving or at best for several months of trouble free motoring.
Lexia gave two fault codes:
03 Sensor power supply
06 Pressure regulation (difference/setting)
The wiring loom from the fluid pressure sensor (screwed to the underside of the gearbox) had been rubbing on the corner of the casing over 15 years resulting in destruction of the protective tubing and wiring insulation, leaving the conductors bare and touching each other! All loom clips were present and intact.
I inserted new wiring, suitably protected and routed to avoid a recurrence. The code 03 "Sensor power supply" did not reappear but the fault remained as did code 06 "Pressure regulation (difference/setting)".
The exact symptoms I was experiencing were documented on the internet. It was stated that the cause is two electrovalves in the gearbox which, during production, were changed from Acutex to Borg Warner. This suggested a reliability issue and was confirmed by Citroën's own technical documentation which also stressed that a gearbox ECU software update is "imperative".
I fitted the Borg Warner electrovalves, changed the transmission fluid and set its level at the correct temperature, reset the "oil wear counter" and, to my dismay, the fault remained. I booked the car into a Citroën main dealer for the software update. With the help of the good folk on this forum it emerged that my car had already been updated as long ago as 2005 when the car was under warranty. According to Citroën's technical documentation this should not have been done since the Borg Warner electrovalves were not fitted at that time.
I returned home from the main dealer still driving in emergency mode.
At times like this I often find it beneficial to reconsider the basics. At the outset, in addition to reading the fault codes, I also monitored the gearbox's live data as I was driving. I was interested in two categories: "Selected pressure" and "Pressure". The former gave a varying value and the latter read "0 bar" until the engine reached normal coolant temperature at which stage a reading would appear. I was suspicious of this but the detailed technical data was unavailable to reach an informed understanding. My electronics background also drew my attention to the pressure sensor which had three wires indicating that it was probably an active device rather than a two wire passive one. An old colleague agreed that there was a chance that the short circuiting wires had destroyed the miniature electronics which we believed were contained within the sensor body. This theory was reinforced by Citroën's price for this sensor: £190 plus vat!
I obtained a sensor for about £40 and fitted it. The result was immediate; correct, smooth operation of the gearbox with no emergency mode.
If you have the symptoms I had, which seem not to be unusual considering the information available on websites, be cautious about rushing to fit the Borg Warner electrovalves (about £100 for the pair from JP Automatic Transmissions). You just might have a faulty pressure sensor.