No rules as such, but they would likely have internal processes/guidelines that the representative has to abide but, but everything is negotiable and escalating to a manager and threatening to go to the ombudsman are ways to seek more flexibility - remember the company will always seek the highest amount first. The likely reason they are seeking to get some money over three weeks instead of in three weeks is that it makes sense from a business perspective to get some of the debt in asap. A company isn't obliged to release its collection/payment guidelines for late bills, just advise in general what happens if a customer doesn't pay.

If the bill wasn't received due to their error with not issuing it - then their payment terms should start from when they issued it - so you would have had the same amount of time to pay as if you had been issued it properly. If however, they claim to have issued it and its just not been received at your end its a bit trickier as there is some onus on a customer to let a company know they haven't received a bill (eg if you know you are billed monthly but haven't received a bill for two months then you are responsible to still attend to your accounts).

The best advice if you aren't happy is to escalate the call to a manager or above - as often the ombudsman will recommend this as first course of action anyway

HTH babe x