Having only just discovered the icalshare service, I've picked added one of the calendars from there to my account on my home zimbra server, worked great, no problems. Thinking that this could be useful in work, I'm finding that it cannot locate the file and it looks like this is due to the work server being behind a proxy server on post 3128 and needing authentication. The home server is behing a firewall that does transparent proxying.
The calendar I'm trying is webcal://ical.mac.com/ical/UK32Holidays.ics and the error shown is:
The following URL is not currently reachable: