Upgrading to newer versions of MODx here proved very problematic due to the size of site and bespoke use of code and plugins - I therefore plumped for a different solution to this issue. I found here
http://www.dangibbs.co.uk/journal/clear-modx-revolution-cache-from-php-script a solution whereby one can create a PHP file to sit in the site directory which when called, clears the cache of the site from outside of the manager - e.g. no need to login to do it each time... I will then work out a way to call this page every few days to auto-clear the cache and therefore remove this caching issue I have...
A bit of a dirty solution... but it works...