I've fixed the problem. The default page width was set at 970, at first I just removed this but it was automatically replaced with a width of 768 (the same as the breakpoint). I then changed the default width to 1920, with a breakpoint at 768 on every page, and it works as expected now.
I'm happy that it works now but still puzzled why the preview and live version behaved differently. Anyway, thank you to you both for your help. I'm very grateful that you are always around
