MicMac - thank you for alerting me to the difference between saloon and estate rear wiper motors. My error was that I thought this motor was the correct part. When I compared it to the new motor, I thought that new part was wrong. The spurious wiper motor does actually fit now, after some tweaking of the mounting points!
Also, now I know the warning signs were there before the original wiper motor went on fire (!) The blade would often get stuck mid-stroke and would tend not to go back to its resting position. This got gradually worse, and then one day the car filled with smoke
So, the moral of the story is: When you see what I described above, time to replace that wiper motor!