Hi
A flex barb is the one on the second link, to connect flexible hose directly to solid tube.
From the picture it looked like the flex was pushed onto the the bit of copper.
So really what you need is a simple 4mm to 4mm connector - http://www.tom-parker.co.uk/products_subgroups.php?v=1...&recall=true&products_categories=7&level2=90&products_groups=1460&products_subgroups=1460AA
any compressed air supplier will have them, you could take yours as a sample.
It would be best to cut of the old olives and start with a new bit of pipe, the olives and/or the tube has been damaged through over tightening.
Alex