Well in the end I sorted it myself, so in case anyone else comes searching this forum as I did, this is what I found.
For three weeks after I posted, the fault continued, and the car started to develop a slight misfire, with the orange light and the "go to dealer" message flashing furiously. This mostly happened whilst driving through the middle of nowhere in rural France in the dark and the rain. Still only the one fault code was being recorded.
By this stage I was getting pretty hacked off. Should I trade it in for something more reliable? Should I drive to the seedier suburbs of Paris and leave it unlocked in an alley so some discontented Frenchman could at least have a bonfire with it?
In desperation, having read on here that a new battery is sometimes helpful, and seeing that mine was still the original, I shelled out £100 for a new one. I was just congratulating myself on fitting this without having to reset the radio code (which I didn't have - did I mention I got this car at what seemed like a knock-down price?) when I leant on the wing to listen to the engine note - at least I leant on the plastic cover just inside the NS wing, forward of the battery. The engine slowed down. Odd! I took my weight off the cover. Engine sped up. Leant forward - slowed down. Removing the palstic cover (which hides the a box of electronics for the ignition), I found that the wiring which should have passed under the end of the cover had been routed under the side and was trapped between the cover and the metalwork where the inner and outer wing panels join. Sure enough the insulation on one of the cables had worn through and it was shorting on the metal. Five minutes and a foot of insulating tape later, job done.
Real cost of the repair about 2p. Money wasted by taking it to the dealer, and trying things that didn't work, about £600.....