It would be great if there was a feature which, once you've spoken to a client and agreed a future payment date, would stop any future automatic statements or debtor chases.

I would see it as follows:

There's a debt collectors box attached to either 1) the customer's account as a whole, or 2) certain invoices on a customers account.

Either way, if there was a "postpone statements, automatic reminder letters to request payment" up to a certain date this would be AMAZING!