I finally raised a call about this issue as the catalogue backups were hardly ever completing successfully. These are the responses so far:-
"I only found one other case in our knowledgebase, and the problem was resolved by stopping all of the NetBackup services, renaming the file to a different location, and then restarting NetBackup. 'Process Explorer' (from Sysinternals.com) was used in that instance to identify that the file was open by the 'nbsl.exe' process.
Backline have confirmed that the file is used to cache job details for the purpose of running NetBackup 6.0 catalog backups, and the file is non-critical (for the purposes of disaster recovery), hence the status 1 on the catalog backup job. To resolve:
1. Shutdown all NetBackup services (ensuring that nbsl.exe stops as well)
2. Move the file out of the var/global directory to another location with a different name
3. Restart NetBackup
4. Attempt another catalog backup"
and
"Glad to hear that the catalog backup went through.
It's difficult to provide an exact reason why the file needed to be renamed, as there was only other one instance of this occurring for our customer base. But the file is essentially a temporary, one, and not required for a disaster recovery restore (of your NetBackup server). Typically, the file should be 0 bytes (or very close to it), so I suspect that nbsl may have been caught in some kind of loop that caused the file to grow in size, ultimately resulting in the catalog backup problem.
If the file grows in size again or you have concerns about the usage of the file, let me know and I'll escalate the issue."
Our file is still growing so I have escalated it.