Well there is a simple way to resolve this.
If they are insisting as progressive payments ask your bank to debit any shortfall from your account for the progressive payment.
It's done commonly in the legal profession for conveyancing for settlement and only requires you to fill out a single form which gives the bank authority to debit any shortfall to the progressive payment from your account.
I haven't had the issue you experience so it's interesting to hear it for the first time. I might talk to Bill one of the managers at Eden Brae who I know to find out what's going on.
Kind Regards,
Bruce
"Building an Eden Brae Wardolf 50 in North Kellyville"
Blog: http://waldorf50.blogspot.com.au