I have concluded that it is not the php files as I have had them replaced them with new copies from an x-cart which is working on the same server.
It is not the database - as I can add through phpMyAdmin.
I am thinking its something to do with Plesk... My colleague uploads files as the 'master user' using 'secure ftp'. I cannot overwrite these files using normal ftp (I do not have permissions) - so I am thinking that this is also somehow causing the problem!!!
It could be, I know that on one of my database servers I have 2 users, one just has select rights, the other has select, modify, insert and delete. So if the same thing exists within Plesk then that might account for your problems.