Thanks for your quick responses.
I have discussed this issue with support during an existing case. It was during this other case that this problem became evident.... I have been advised to open a support case for this, however I left on annual leave today and will not be back at work for 12 days or so. I will open a case as soon as I return, but I thought this might be worth discussing here to see if others have seen a similar issue.
The tape Library is a HP MSL2024 with LTO 4 drive, attached via FC. The write speed of 120MBs is really about spot on for the hardware, no problems there.
Backup server is physical HP DL380 Gen8 using direct FC path (8Gb), not hot add of course.
The backup and restore jobs for this particular issue are all local to the Veeam backup server, ie the backup job respository is saved to FC storage directly attached to the backup server, then the backup to tape job is from FC storage to FC attached tape library. So the only proxy involved is the local instance. All storage to VM's, tapes etc direct FC attached to backup server.
During the restore, the FC library is restoring back to the same FC attached repository on the Backup server when it originated, so the path for backup and restore is identical.
No hardware encryption or compression activated on this test backup.
Again thanks for the prompt questions, I will raise a ticket when I get back from annual leave, but until then, I hope this gets some discussion, I would be very interested to hear from others.