I spared you the full story. The problem was water leaking into the motor gearbox, caused by a faulty seal between the wiper spindle and the plastic elbow/bush where the tube from the pump is attached. Eventually, following a tip from a post on another site, I fitted an O ring from my super selection box, although there was no trace of one having been fitted in the first place, or a groove in which one would fit. But there was space and it seems to have worked. The cure was preceded, however, by a great deal of hunting for a replacement motor. The pre 2012 model (8P) seems easy but 2013 on (8V) seems impossible. Used ones were available, though- at a price. I also checked to see if the wiper arm would free easily if I went for a replacement. It wouldn't. My puller didn't fit and I suspect even with one that did it would have been necessary to destroy the arm, adding another £40 or so to the bill.
So I feel I've got off lightly at £15 for the trim tools and, probably, 5p for the O ring. Until next time!