Just thought I would update my post with a couple of solutions....
The backup was failing to restore due to a couple of differences in the 2.6 and 2.4 servers.
Turning off the Development, Experimental 2.6 backup option enabled the courses to correctly restore in version 2.4
We then experienced an issue where the books were not being imported. This was due to character encoding differences (UTF8 on Moodle 2.6 Server and Latin on Moodle 2.4 server). Now we have changed the 2.4 server to use UTF8 the books are also importing.
I hope this helps anyone who may experience the same problems.