Bootcamp only option is windows 7 not windows 8 etc on late 2013 model

Hi

I have a late 2013 model imac, I need to run windows on part of the partition for sage accounting software, when I launch bootcamp assistant the only option I have is to install Windows 7 and nothing else, I have downloaded all the Apple updates including updating to El Capitan OS X to see if that helps but have the same issue.

Have tried to install Windows 7 but have the same issue as most that the keyboard and mouse don't work on load.

Any help appreciated

Many thanks

Paul