Hi GraemeW,
Thanks for your update. I've done a lost of tests on my side. I've tested on several Office 365 accounts, the URL you provided doesn't work on all of them, but if I add the calendar URL on my Outlook client or Hotmail account, it shows fine.
Based on my experience, it may be some incompatible issues between iCloud and Office 365. Given this issue, I've reported to our Office 365 backend product team. If there are any updates, I'll post here.
Additionally, I also recommend you submitted feedback via the link below. Your comments and suggestions will help us improve our service. Thanks for your contribution on this thread.
Feedback
http://mymfe.microsoft.com/Microsoft%20%20Online%20%20Services/Feedback.aspx?formID=210
Best Regards,
Aaron Yuan
