Thanks for the answers. I'm now on 10 and (surprisingly) the upgrade did just work. It didn't even try to force me to use an MS account (although it did notify me that not having one was an issue, I could just dismiss that notification). Unsurprisingly, it took ages with little or no indication as to whether anything was actually happening. Including several minutes of nothing but an image of an admittedly very pretty landscape for some reason. But now it's on and I've used WPD and Spybot Anti-beacon and think I've reduced the worst of the spyware.
So now I just have to wait until MS force updates on me that break something. Yay, the joys of Windows 10.