I have then tried manually updating from the Apple website database - doesn't work. I have also tried looking through App Store and manually updating that way - doesn't work.
I have tried via the classic Software Update route but nothing comes up. I want to update it as I appreciate that El Capitan is well out-of-date and my apps (and system in general) is starting to reallllllly slow down.
I currently have an iMac (20-inch, mid 2009) running on OS X El Capitan (version 10.11.6).