Thanks Travis,
Yeah I just found that it didn't pull the update.key file from the VSAHiddenFiles\kav\ dir on my SaaS. I thought it might have just been a security thing where SaaS users have no rights to these dirs.
I ended up finding an update.key on a system I recently installed KAV to though and I uploaded this to my managed files to use in the script instead.
All works perfectly now thanks.