Thanks for your reply. I didn’t add any code to those bottom areas. There was only a search box widget. I managed to fix it by creating a new home page and rebuilding it. Now the width appears as it should, but I can’t for the life of me work out why it played up like that. I will explain what happened anyway, for the interest’s sake.
I created a fresh page with no content and the footer appeared within the boxed area. The original home page was constructed using PageBuilder (Site Origin) plugin, so that I could have 5 columns as well as the a slideshow in the Banner Wide position. I then loaded the prebuilt PB layout cloning the original home page. The footer and bottom area jumped to outside the boxed bounds. I scrapped that and then created another new page, adding the content in PB row by row (instead of cloning) and it now works fine.
So obviously there was something strange going on with the PB rows that was invisible to me..