This issue has now been resolved. With Alan's help, I was able to narrow the issue down to a Kerberos authentication issue, caused by the fact I never added the registry key - described in this article - to the machines. This prevented me from connecting using the AMT WebGUI but also prevented the alarm clock from being set correctly. Once the registry key had been added & PC rebooted, I was able to connect using WebGUI and PCE alarm clock was set without issue.
Thanks again for all the support Alan.