I've actually JUST resolved it.
I use both my Laptop screen along with a second 22" screen where I display the applications when working on them. Earlier today I changed the relative positions of the two screens along with the settings in display properties.
I would have thought that the changes would be reflected or if the screen location was not addressable then revert to the laptop screen. It reverted to the laptop for full screen but not for partial screen.
So, I moved the properties back to there they were (approx), tried again and it worked - so left word open, changed properties again and the window moved across! Does that make sense to you?
The Regedit suggested is a little drastic - there must be a line giving the default co-ordinates that I could have tweaked! I may try that if I get another app which has issues.
M H C
taurus excreta cerebrum vincit