Update: I have since explained this to Mitel with no real feedback apart from, "you'll have to configure it from scratch",huh? they can't be serious!
I have done a manual upgrade, was able to log into the ESM with the default credentials, but unable to retrieve licenses as the controller does not have internet access, so I reverted to using Software installer to download the licenses and perform the restore, this worked but SI gave me an error saying that I should check that the user had sufficient privileges, obviously this was due to the default credentials not matching those in the backup. Once the restore was complete, I was still unable to access the system either via the original password or the default. The only thing I can think happened here was that SI was unable to complete the whole process, setting user restrictions so forth hence the lockout.
My next course of action I will try is to do a manual install, retrieve licenses via SI and perform the restore via ESM. I will keep you posted but would appreciate if anyone has a idea on how to reset the password without having to perform the manual install.
Thanks