How to restart a getMail backup from scratch?

I recently installed getMail and configured it to backup my gmail account.. No problems.

After subsequently reading through the getMail configuration docs I decided that I wanted the mail delivered to a Maildir rather than a Mailbox, so I deleted the mbox file and log, and I reconfigured the getMail config to specify a mail dir.

My problem is that after initially backing up the account then changing my getMail config, getMail subsequently returns '0 messages (0 bytes) retrieved, 0 skipped', with or without the -a, or -all command line option, or with/without the read_all = true option in my config.

It seems that getMail (or gmail?) has cached the fact that messages have been backed up?

Mmy question is how do I trigger a *new* backup of all messages to occur after having partially or completely backed up the account previously?

Tq,