Update. I took the car back in on that Wednesday. They couldn't find anything wrong apart from the battery being slightly under voltage. They asked to keep it overnight to give the battery a good charge, which I let them do. Next day I was told that the only problem being reported was still the same undervolt, so they were going to replace the battery at their expense because the battery problem wasn't being flagged as a problem by VW diagnostics and wouldn't be replaced under warranty.. I was told they didn't keep batteries in stock and that I'd get a new one the following week. Bo real problem with that as the car was perfectly usable without stop/start. So I drove the car away. Guess what, the start/stop was working. I could see by some of my setting that there must have been some sort of general reset performed so maybe that cured the issue. Goes to show they didn't bother to really test the stop/start, but just relied on computer readouts. Worrying. Anyway I drove the car for a further week with the stop/start working perfectly, but I thought I might has well have a new battery for all the inconvenience they had caused me. That was replaced yesterday. Stop/start is still working. Incidentally, despite all the warnings about various parameters having to be right before it would work, even in the cold snap we have just had, the start/stop is operable within about 30 seconds.
So, nobody seems to be any the wiser as to what was causing the problem and what fixed it. Hey-ho. The cars are too clever for the technicians, I guess.