Ok, so we've tracked down the problem to be that our host doesn't run CDONTS on the 2008 server.
They've asked us to select JMail instead, but they are suggesting that we don't need the username or password in the settings?
Anyway, the UE wizard requested those details so I assumed we were going to run into problems but it seems to be working without specifying a username or password. Does this matter, particularly?
Hopefully this thread will be of some use to someone! :o)