When I then enter sudo mount -a it works as expected.
However, after a reboot, the NAS is not mounted and Dolphin shows the error message, mount.cifs: permission denied. When I open Yakuake and enter sudo mount -a, it works again.
This suggests that the line in etc/fstab is correct (or very close) but why does it produce the error so that I have to enter sudo mount -a after every restart? If anyone could tell me what I need to do to get this working as required, I would be very grateful. Thank you.
Thank you all for your fast and informative replies. If anyone else find this thread with the same problem, reading about systemd and a quick experiment proved what causes it. The WiFi networking is too slow starting up so the path to the NAS is not available when the fstab entry tries to mount it. A quick test proved that the error does not occur when the laptop is connected to a cabled network.