Thanks for your advice. You are absolutely right. This worked perfectly. I guess that I was expecting to see the same kind of behaviour in Outlook as was apparent in the web.
I had a slight aversion to opening another users mailbox (from a security point of view), but obviously Zimbra handles the security aspect of this, so only the calendar (which is shared) is visible to the user opening the mailbox, so this gives me exactly what I want
Thanks once again
Andrew