Hi @Northy, sorry to hear about this.
Does this happen when you try to update the software using the phone settings, when you try to update using Xperia Companion and a computer or both? Also, have you tried clearing the data for the system app in charge of software updates? This will reset all the settings for the app which might do the trick. Find and tap Settings > Apps & notifications > Show all XX apps > Tap the menu icon (three vertical dots) int the top right corner > Show system > Software update > Storage > Clear data > Tap "OK" > Restart the phone.
Another thing that you can try is to reboot your phone in safe mode to see if you are able to update the software there. All apps downloaded from Play store is temporarily deactivated while in safe mode and by doing this, you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
Try these measures to begin with and let me know how it goes!
