I have removed the 4.27.1 install from the launcher Remove option.
I am now reinstalling it, and so far it is downloading the 6.3Gb.
(It couldn’t download 3,6MB for a repair lol)
The ONLY thing I can think of right now is the 4.27 was installed when the vaultcache was on a different drive and it is getting confused attempting this repair download, who knows.
The other thing I did see in my investigations was in a log file there did appear to be some downloads failing from a host of Epic download locations. There is no reason any such domains would be blocked/black listed given I am having no trouble with plugin updates and other engine updates. Removing it and reinstalling it is working (so far) so clearly it can reach those sites.
I will close this if the install completes, but an explanation would be useful for the future.
Cheers.