Okay, so first attempt at downloading, it got to 60% and stopped giving an error that said
Unknown Error: E-Fail
So I cut off the proxy in the options at the recommendation of a friend who'd had similar problems.
Then it got to 88% and gave the same error.
So I ran the repair tool
Now it gets to 88% and the launcher just closes.
I've now uninstalled and trying a fresh download.
Assuming it doesn't work, I wanted to post here and get recommendation for any further steps.