I've noticed there have been a lot of unresolved threads regarding the plugin not working, and a message regarding permissions of 755 or above required for wpconfig. On changing the permissions, (and for wp-content and the cache folder) the plugin still doesn't work.
However, the path detailed in the error message is for the hosting root html folder.
So - if the wordpress installation isn't in the root hosting space, the plugin won't see the wp-config file, regardless of the permissions.
In my case, the client website was on one hosting space, which features multiple websites. On changing the permissions for the wp-config file in the root (ie a different website to the one I was working on), the instance of quickcache for website I was working on (located in a client folder in the root) then started working. The plugin was looking at the permissions for wp-config in the root, and not the actual wordpress installation.
Is there a fix for this?