PC Hardware Diagnostics UEFI ver 8.0.0.o gives black screen

[karborundum]

I Have a ZBook 15 G2. It has recently been completely reset and reinstalled from scratch, including new disk. BIOS and firmware updated as well as everything else that Support Assistant suggested. Unfortunately this included the Hardware Diagnostics (Windows v 1.6.8.0 / UEFI v 8.0.0.0).
BIOS settings are "default" which could be relevant as this is a dual GPU machine (Intel 4600 / NVidia K2100M).
The windows part installed without complaint and works fine. But as there are some tests than must be run from UEFI-boot, I successfully installed it on the hard drive, at least no complaints. Have retried installation it since.
Problem is when I boot the computer, usually via Esc to get the menu, every other option works OK but the F2 - Diagnostics hums a while and then gives me a black screen. Nothing more and nothing less. Tried going directly to F2 start, no change.

So I created a USB version and started via both Esc and F2, same result, a black screen. The USB flickers so I can see it is reading the drive. Tested the USB on an EB 8570w, same black screen.
Downloaded version 1.6.8.0 and tried to install, the ZBook won't let me downgrade, "newer version exists / Exit".  It won't even give me the option to create a downlevel USB installation!
OK, I fixed a USB on an EB 8570w, tested it and it worked there. Took it to the ZBook, it worked perfectly.
So there is an issue with the new version which I should have suspected, no, rather expected with version a X.0.0.0.
I'm perfectly happy to stay with the version originally released with the ZBook. And I will not upgrade the ZBook G3 I've been setting up.
So, for me the problem is :
- how do I downgrade in a nice way?
Wiping the recovery partition does not make me very enthusiastic.