Cannot get 404 error page!

Hi,

I am facing a very unusual problem. I have customized the 404.php page and although everything was working fine for weeks I have recently realised that my 404.php page doesn’t work anymore. If a user requests a page that does not exist instead of being redirected to the 404 error page he gets a page that consists of all my sites pages! I don’t know what broke it, maybe one of the plugins I have installed or something else? I really can’t come up with something creative. Any help would be great.

Thanks

My site can be found at https://studentnetworksussex.org.uk/cms/ . Please try to request a non-existen page to see what happens instead of getting a 404 error page.