Ok I have done some more digging and it looks like there may be an issue with the call to
http://home.mydomain.com/service/soap/BatchRequest
Firebug is showing a 404 at this request and the message was
"The requested URL /service/soap/BatchRequest was not found on this server"
I have added a proxy rule for /service to also point to the zimbra.mydomain.com and now I get a custom 404 in Firebug. So it looks like my proxy rule is some way to resolving this.
Any thoughts.
I am going to continue digging but any pointers would be appreciated.
Cheers
Tim.