It looks like I have might have sussed the cause of "random rotation" on Windows 10, just in case it ends up affecting anyone posting pictures on here.
It seem that taking the picture portrait or landscape as required with a phone or tablet is fine. The problem starts when the pictures are sent from that device to your PC.
If the tablet isn't in the same orientation when you upload as it was when the picture was taken, Windows 10 sees it as rotated

even though you will see it as "normal"
I have added "Orientation" in the file view and this is where our PC guy spotted it says "Normal" or "90 Degrees" "270 Degrees" etc. Even when it looks as intended.
Any files that say anything other than "Normal" I right click and open in Windows Paint. Here it will look as it should despite any rotation info, I then just click "save" and close Windows Paint.
When you see the file info it will now say "Normal" and upload here, Facebook, wherever without rotating at random.
This isn't a cure but is only a three click fix that's quick enough to put up with until Microshaft sort it out.

James.