Thanks Grant. I'd tried all the basic stuff. This is a work laptop, of
which we have two (both encountered this problem). Our IT Dept are not
much cop when it comes to certain software problems so I managed to get
admin rights on a standalone (off-network) userid.
What I discovered was multiple virtual Epson projector monitors (3 in
all). This would explain why on occasion I had to drag (using keyboard
shortcuts) an application window two or three screen widths before it
would appear on my laptop screen!
Anyhoo, I could not disable these in the usual manner so I removed them
using device manager and hey presto, I'm back in dual-monitor heaven.
I should have posted this late last week when I resolved the problem,
but it went clean out of my head.
Apologies and thanks again for your time.