Writing this from W10
WUD has still been doing "failed" attempts when I left 8.1 running overnight (80240020) but a little delving around revealed that I've had the $Windows.~BT hidden download file on the system all week.
I can understand MS saying "wait, we've got a huge queue waiting to download", but I completely fail to see why they said "wait, you've got it all, but we're still not going to let you start the upgrade yet"
Turns out a simple "EnableOSUpgrade" key inserted in the registry was all that was needed, and the previously downloaded 4Gb file switched on and changed 8.1 into 10 in next to no time.
Don't understand the thinking behind that at all !