Hi Paul.
Sorry you’re having this issue.
I tried that iCal URL on my local testing site and saw this error message: “Unable to retrieve content from the provided URL.”
However, I checked all the things I could think of and didn’t see any reason it shouldn’t work.
Turns out that the issue is due to iCal Importer’s default 5 second timeout.
Here’s some code to change it to 10 seconds, which made it work for me:
https://gist.github.com/cliffordp/35dbf3ca36c57949228bf6ded23d62bf
You can choose the number of seconds you think best but please don’t put it too high (potential for other conflicts).
Please let me know if this solved it for you too.