Finally, I have got to the bottom of this with the help of Vodafone and Wireless Logic.
There is a lot of misunderstanding around Mail Connector and how it works, mainly because it is useful for individuals rather than corporates and therefore of limited interest to support companies. RIM, who authored the software, are hopeless and won't talk to you (or even Vodafone in my experience). The initial reaction is therefore to say that it is not supported or that it won't work for you for some reason.
If, like me, you've been using it for some time then that doesn't sound very convincing!
Anyway, the answer is that there is a bug in the update and RIM are so cluelesss that after several months they are no nearer to sorting it out. You need to uninstall and re-install Mail Connector. The problem is that there is nowhere you can download the file - it has to be done as part of the setup, necessitating a complete re-hash of your setup.
The procedure is:
(1) back up your phone data (in case)
(2) note down all your email account settings on mobileemail.vodafone.net - or whatever your carrier's site is called
(3) delete the email account on the site
(4) uninstall Mail Connector from your computer using Add/Remove Programs in Control Panel
(5) re-enter your email details on the website, opting for “Work address” and “Always Use Outlook” when asked. This will prompt the Mail Connector Download.
Note that you must be using Internet Explorer and an Active X control must be allowed. If you get a message about not having enough rights then ActiveX is not being allowed.
You should then be up and running in 20 mins with the new version of Mail Connector ... and no update messages.