Found my own solution, may be obvious to you professional but thought I would share it.
There appears to be a huge inconsistency in the way that the different LTI links work, on the different versions of Moodle. Some versions of Moodle, will accept the links as we created them in Moodle 3.3.1 as the cartridge Plugin. We have discovered that some versions of Moodle will not accept these cartridge links and they will only accept a more basic link, based on a simple URL
So if the cartridge link, fails then to manually add the enrolement url and secret. If you do use this method you must open the 'see more' link and add a word to the consumer key. Without this key, the link will not work.
Hope this helps.