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Thanks for your reply Rojash. No, I haven't set the view to Page Break Preview, it's on "Normal". (I have just tried to see what happens with Page Break Preview and, as you say, it changes background colour to grey - and it also reduces cell size somewhat. I've changed back to "Normal" and still don't have any grid lines.)
I've uninstalled and reinstalled and still have the problem.
Is there not a checkbox somewhere, as in Lotus 1-2-3, that enables you to toggle between showing and not showing grid lines? I know I can format cells so that gridlines will print but I was after something "global" by default.