Thankyou everyone who has posted suggestions on this and other threads!
I was getting error code 0x800F9022 with a full system reserved partition. What hurt was that it took an hour to install to the point of failure, then a further two hours to undo all the changes.
An MS tech support level 2 guy looked at the error code and checked my BIOS version. He then went on to say that the BIOS was old and that a UEFI BIOS was required in order to install KB2919355. Rubbish I said, my other computers upgraded fine with a Legacy
mode BIOS. He didn't even look at the log files or check the system reserved partition.
What worked for me, was doing the "Marek Kusmierski" process
in full, including renaming the SoftwareDistribution folder pre-process detailed a few posts later, followed by increasing the size of the system reserved partition as detailed here. I then did a manual check for updates using Windows Update, and successfully
got it on. After downloading, the installer completed all phases in about 30 minutes.
It was then just a case of rebooting a few times (just to make sure), and then manually cleaning up all the junk left behind.
Thankyou so much!
Addendum: The computer was also in a "clean boot" mode at the time as well.