OK, the problem appears to be with the lack of an aliases.db file. Even though an alias was not needed, as I was mailing to the direct account name, the fact that I had not set up an alias file and compiled it into the aliases.db file (as referenced by the sendmail.cf file), caused all local mail to be queued.
Outgoing mail still went fine though.
As a side question then, where does sendmail (or procmail, if it is really being used as the local delivery agent) actually store the local mail files for a particular user? Can this be configured without a .procmailrc file? And which is better for a small system, a .procmailrc in each account, or a global one?
Thanks up front, for any advice;