Garmin Express told me I had updates for my Zumo.
There were two updates, one for "safety cameras" and one to do with time-zones.
When I told it to update all, it said "this may take a minute" and stayed like that.
After an hour I decided that given I had a backup, I would take the chance of corrupting data, and closed Garmin Express.
I repeated the exercise with the same results. After googling the problem and getting no solution, I tried doing the two updates one at a time. It worked.
Hopefully this may help someone else in a similar position and
hopefully it may help Garmin reproduce the problem.