'No sidebar' template not working, but working!

Any help would be greatly appreciated.

One of my ‘no sidebar’ templates isn’t working correctly – the sidebar is still showing, but over the TOP of the content on my page. To check if the template is corrupt in some fashion, I moved it over to a test WordPress install. The template worked, which has proved that perhaps it isn’t the template that’s the problem. All other templates appear to be working without issue.

Here’s the page which is affected: https://www.wolverley.worcs.sch.uk/school-calendar/

Any ideas?