So when it failed again, only about 2 months after the last fix, I thought I was in for another expensive fix. preceded by a £120 charge even to run any diagnostics.
The car was usable but very under-powered - getting up slight hills was a major exercise at about 20 mph :-(
I booked the car into the garage, then something made me check under the bonnet. Lo and behold, the turbo hose had come off :-( It took about 2 minutes (one of which was finding a screwdriver) to loosen the Jubilee clip, refit the hose and tight the clip. Normal service has been resumed - the car can actually go faster the 50 mph again and can climb hills.
So my poor car was running as a normally-aspirated (non-turbo) diesel, with any changes that the engine management system may have made to cater for this situation.
The warning light is still on, but I wonder whether it latches on until it's manually reset by a garage. Hopefully the garage will reset the light for a nominal charge. I wonder whether the work they did on the car 2 months ago involved removing/refitting the turbo hose.
(*) The total dying of the engine happened in the weeks leading up to moving house, when I was making journeys over to the new temporary house (my parents' holiday cottage) with cardboard boxes - wonderful timing. Each time it happened after I'd driven about a mile from home after starting from cold. I got used to restarting the engine, driving about 20 yards before the engine died again, repeat about five times and then the engine stayed on and the car ran perfectly for the next few days.