Sorry, please mostly ignore my last post.
I going to make myself a new rule. Let problem sit for 24hr before posting to forum.
Something very strange was going on. My Buffalo uPD720201 card appeared to be dead.
The other ports were clearly misbehaving.
So I shut everything down. Both the Test PC and FX3 board. I left it all completely powered off (at mains switch) for a while.
Then I removed the now dead uPD720201 card and replaced it with a new uPD720202 card. And rebooted Win8.
Now everything is working. All ports can see bootloader. All can download firmware. And in all cases the firmware works.
Performance profiles still look very different to Win7 x64 though (peak performance is at 64K xfers. 256K go slower)
But I've only checked EJ168 and uPD720202 so far. I'll check the rest of the controllers tomorrow.
The lovely 4-port uPD720201 card remains dead unfortunately. But at least my FX3 dev board is OK.
the MSDN is open for win8