Many thanks Bharat. This led me down a trail which ultimately identifies the
underlying issue as - Migrating Ex5.5 to 2003, the DLs on 5.5 used for ACLs
are migrated to Universal Distribution Groups in the AD, but *only* if the
AD domain is in Native mode at least. If it isn't, the ADC generates errors
(and presumably the installation fails). So, now I know what the underlying
issue is. (thatnks again!)

Next on my things to track down then is, is it possible to raise the level
to Native with an Ex5.5 environment (obviously after any NT BDCs have been
decommissioned) or can it only exist in a mixed-mode domain?

If the former, then we can ensure that, on the day of the start of the
migration, we "freeze" the environment (email & account changes etc.) get AD
installed, get BDCs decommissioned, raise the domain level, THEN install the
ADC for 5.5 and sit on it all for a while to ensure things are running
cleanly before we then introduce the new Ex2003 server and start migrating
mailboxes/ Public Folders.

Does this sound do-able?

Regards,

Paul