Ford Escort 75 van 1.8td how to repair an immobiliser system

I've had this van from new and had all the services done to the letter, even changing the cam belt earlier than suggeested.
It's been fabulous and never let me down. Then it stopped, just going around a roundabout and onto the road and it felt as though the engine had been switched off, no power when gas pedal pressed and no response when I tried to restart it, nothing not a cough. The hazard warning light s were working as was the radio. Mechanic here (in France) thinks its something to do with the immobilisation system but doesn't know what, or how it might be repaired! Any ideas on possible cause or repairs please?

the battery is almost new, and there was a full tank of diesel so its nothing simple (if only!) This is a bit of a rural garage and probably not totally up to date. Its not possible to move the car so I am totally in this guys hands. Any help would be appreciated jan smith