9100 - BIS not synchronising old emails

[AdeptIT]

I recently moved countries, and so changed providers from TmobileUK to Optus in australia.


i had BIS setup on my 9100, and pulling down my emails. It worked perfectly.
While in the process of changing over, I didn't consider the situation, and configured the new optus BIS account. It re-downloaded all my emails.
This put me in a position where I had duplicates of around 300 emails. Due to the pain involved in deleting the duplicates one by one, I decided to just use desktop manager to wipe the emails )after backing them up first(

I then removed the email account from the BIS webpage, and re-added it, expecting my emails to download.

Now, I'm only receiving new emails. It won't resynchronise the old ones, and I cannot find any way in which to force it to do so.
I'm concerned it may be similar to BES, which knows it's already downloaded those particular emails and so won't resend them.

Any hints on how I could sort this out? (It's using an imap server, and is not Microsft Exchange)