If you are earning income and do not pay your ISA, after a certain number of days we consider you in default. This means you owe the entirety of the Prepayment amount and we will pursue that amount to the fullest extent of the law.
If you cannot pay because you are not earning enough, that's fine. Each ISA contract has a minimum income level at which payments start. If you do not earn enough in a given month, we do not collect any payments.