Found a serious problem in Windows 10 Insider Preview build 14342.

If you run:
dism /online /cleanup-image /startcomponentcleanup
the component store becomes corrupted, and this results in Windows Update failing to install updates with error 0x80070643.

Unfortunately, I have not yet found a way to repair the component store.

I am backing up my data, and preparing to re-install W10IP 14342. For the moment, anyway, it appears that the only solution is to avoid using the above command.