@Steven
It was only happening in Chrome. Mobile Safari was fine.
I can't change the meta viewport tag, so I hadn't done that.
I tried your suggestion of setting the expandInNewPage value to TRUE and FALSE to test.
It seems to be fixed now! But it was fixed by using expandInNewPage="FALSE", not TRUE. Your earlier response had said that it should be set to TRUE, but the opposite seems to have fixed it for us. I checked it in iOS6 and iOS7 and both seem to be working. It seems to be ok in all browsers too.
Thanks,
Yvette