I left the PC "Preparing to install" overnight. This morning there was a screen:
"AvastUI.exe - Application Error
The instruction at 0x0000000075E181B8 referenced memory at 0x0000000075E181B8. The required data was not placed into memory because of an I/O error status of 0xc000000e. Click on OK to terminate the program."
I had to reboot after this.
This might mean that there's some kind of a conflict between my Avast AV and these W7 updates, or be entirely unconnected.
There are two important updates (in addition to 6 optionals) which have got stuck:
1: Cumulative Security Update for Internet Explorer for Windows 7 for x64-based systems (KB2929437) - 45 MB
2: Update for Windows 7 for x-64 based systems (KB2904266). 729 KB
1: may of course be blocking 2:. Both appear to have been downloaded but not installed.
The system is set to receive updates automatically at 3 am (when the computer is normally switched off)
It looks like the vast majority of previous updates since I switched recently to W7 have been installed successfully. I have rebooted several times since then.
Other than to repeatedly run chkdsk /r again in the hope that it unblocks the jam I am unsure how to proceed.