A few days ago I'd been trying unsuccessfully to get a DVD encoded from a disc image file, using the Men's Shed iMac. Then, when shutdown time came, the Finder kept asking if I wanted to quit the encoding, quit it without saving or cancel. It made no difference what I chose, the window kept popping up & shutdown was aborted each time.
Fuck this, I thought, time for an Option key trick. Go to the Apple icon on the menubar, select Shutdown with the Option key held down. This cuts out the bullshit dialogue boxes and shuts down immediately.
Fixed.