We checked the volumes today within the raid manager, all the volumes were set to write back with BBU (battery backup), the raid manager says the battery backup is present and it doesn't report any issues, we changed the volumes to "always write back" and the storage is now fast.
I will get in touch with IBM to see if this is a bug, the firmware of the controller was upgraded recently to try fix this slow issue but it didn't make any difference, I know it's not recommended to leave the volumes as "always write back" because of the data corruption that can occur with a power failure if the BBU is faulty or not present but this is their DR server and it's useless if Veeam cannot write to it at a resonable speed.
For others with the same issue test your storage first, make sure that it really isn't the issue, if you are testing by sending data to a Windows VM that is located on the storage send it a 50GB file and see if the transfer speed is maintained, for me the speed would drop off after 10GB was transferred, towards the end of the transfer it would only be transferring at 6 - 8 MB/s.
Try a product like Veeam to upload to the storage and see if that makes any difference.