Okay, after playing around with some testing domains I found that this works.
imapsync --host1 xxx.xxx.xxx.xxx --user1 firstname.lastname@example.org --password1 xxxxxxxx \
--host2 yyy.yyy.yyy.yyy --user2 web6_username --password2 xxxxxxxx
xxx.xxx.xxx.xxx is the older server IP number and
yyy.yyy.yyy.yyy is the new (ISPConfig - Ubuntu 5.10) server IP number
is the user login on the old server and
web6_username is the user login on the new server.
I tried IP numbers rather that the domain (mail.domain.com) as the DNS records have already been updated to point to the new server on my test accounts/domains, but not yet on the client domain. This will allow me to apply for DNS updates and migration over the weekend while client is not inclind to check mail.
I've not done the client mail yet, as he has a number of custom folders and need to see if those will be moved across as well.
But for a basic migration, the above worked fine for me. I hope this works helps somebody else.