Can't install Windows 8.1 Update - Error 0x800f081f

My laptop came with Windows 8 pre-installed. I installed all Microsoft updates, including that to 8.1 and have continued to do so.

The latest update which has come through is KB 2912355 which is a massive 890 MB. This has refused ti install, despite many attempts.

The system reports error 800F081F. Having looked at Microsoft online support, I have tried the following :-

DISM.exe /Online /Cleanup-image /Restorehealth

this failed with the following comments :-

The restore operation failed. Either the repair source was not found or the component store cannot be repaired.
Error: 0x800f081f

I have therefor tried the next suggestion under 'repair a Windows image' :-

Dism /Online /Cleanup-Image /ScanHealth

This reported :-

The component store is repairable.
The operation completed successfully.

I next ran :-

Dism /Online /Cleanup-Image /CheckHealth

This also reported :-

The component store is repairable.
The operation completed successfully.

I therefore next ran :-

Dism /Online /Cleanup-Image /RestoreHealth

The output was :-

The restore operation failed. Either the repair source was not fou
onent store cannot be repaired.
Error: 0x800f081f
DISM failed. No operation was performed.
For more information, review the log file.
The DISM log file can be found at C:\WINDOWS\Logs\DISM\dism.log
C:\WINDOWS\system32>Dism /Cleanup-Image /RestoreHealth
Deployment Image Servicing and Management tool
Version: 6.3.9600.16384
Error: 87
The cleanup-image option is unknown.
For more information, refer to the help by running DISM.exe /?.

I did have a look at the DISM help, but the options are so many and so complicated that I didn't get anywhere.

According to Microsoft, I have an image which is repairable, but nothing they have suggested worked!

I'd be very grateful if anyone could suggest what to try next!