Software Update errors

I have a problem with installing system software updates through System Preferences → Software Update on an iMac running 10.14.6.

When I upgraded the MacOS from 10.12.6 to 10.14.6 I found that there were three updates needed doing still in Software Update. The 10.14.6 Supplemental update, a Safari update and a printer update but when I tried to install them I got an error message, see screenshot below.

I checked the System Preferences → Date & Time to make sure that was set correctly which it was. I then tried doing a Safe Boot and installing from there and also installing from a different User account but got the same error message.

I found an installer for the printer software that I had previously downloaded on another Mac so copied that to can external hard drive and successfully installed that from the external HD. I then downloaded the 10.14.6 Supplemental software update from the Apple website and tried installing from the DMG but that reported an error with the installer and failed, so I downloaded it on a different Mac, copied it to an external drive and tried installing it from there without success.

At this point I rebooted into the Recovery Partition and ran First Aid from Disk Utility before reinstalling the MacOS. When this had completed I tried running Software Update again but got the same error message as above.

I went back into the Recovery Partition and disabled SIPS through Terminal, rebooted the Mac and deleted the contents of the /Library/Updates folder. I went back into the Recovery Partition and reenabled SIPS through Terminal before rebooting and trying Software Update again. Something had obviously changed because although it failed again I got a different error message, see screenshot below.

I’m now running out of ideas. Does anyone know of a solution to this problem?