Thanks Mohit. Resetting the preferences looks quite a complicated procedure. I'll try it next week when I have more time.
It's not complicated. It involves deleting one file, with Lightroom closed.
No luck. I closed Lightroom and deleted the preferences file. Then I carried out a system restart and reset the preferences by holding down the shift + option key when I opened Lightroom, all as per instructions. It has made no difference. The Import process still hangs.
Do you have any other ideas how this could be fixed?