macOS 10.15.1. Software Update won't pick up upgrades

I'm running macOS 10.15.1 on a macBook Air, 2015 model, and I need to upgrade to the latest version.

There are upgrades available, up to 10.15.5, but when I open "Apple Menu > About this mac > Software Update" I get the following...

I know there are newer updates.

I also downloaded and installed the updates from Apple Support, but I always get the following error.

I have:

1. Plugged into ethernet
2. Plugged into power
3. Restarted in Safe Mode
4. Flushed the SMC, PRAM, and NVRAM
5. Tried with and without a VPN
6. Tried with most of the intermediate DMG
7. Deleted some files in /var/db/crls
8. Tried sudo softwareupdate -ia
9. I've got c. 20Gb of disk space

Any help gratefully received

Make sure you use the combo updater if you are trying to manually update by more than one "point" release - 10.15.6 is the latest - https://support.apple.com/kb/DL2048

For future reference, Google can always find these very easily - just search "combo update 10.15" etc & the latest will be the first result.

The combo updater is capable of jumping from any point release to the latest.
It may at the same time fix your Software Update control panel issue, though you may not find that out for sure until the next update.

If that still doesn't work, double check your free drive space. Use Disk Utility & the "Free" value rather than the "Available" value.