You're right, I disabled loads of stuff last night, and the problem frustratingly remains.
I'd ideally downgrade to 4.2, because I think 4.3 is where Sony removed the screenshot option (before cunningly adding it back in 4.4 as a "new feature"), isn't it? I use that a lot, because my eyes aren't that good, and when you hit a website that blocks pinch-zooming, or you get an app that has text that is too small, like Google Earth, I frequently need to capture the screen and zoom in.
Of course, the problem with this solution is that downgrading to a specific rom version, for a specific version of the tablet, in a specific territory, is not that simple. I'll have to spend some time collecting the required files and instructions, then wait for a good day (mentally), to give it a go.