So that's it then
I've been wondering why my monitor's been doing this for ages... every time I stand up it goes blank for about 1/2 a second. I've spent ages checking and rechecking the cables, I thought one might have been loose, but obviously not. I'm suspecting it's the DisplayPort cable that's the cause rather than the monitor itself... but that's easier to check.
It's a relief to know the reason.