I am not sure that Trudi meant the client had suggested she write-off a payment.
But even if it was somebody else, you should never write-off payment for work you have done. At the very least, this would compromise all future payments!
It is true that most commercial, contractor payment runs are made on a monthly basis. You should take this into account before commencement of any contract. At the contract negotiation stage, find out their month-end date. (this is the date you will use for your invoices) Then set your terms at 30 days for full payment. But you must be slightly flexible here.
For example: Many developers use the last Friday in the month - as the month end (except December) This month it will be 28th September. Their payment run is usually the first Friday in the month - so payment for the work done in September will be on Friday 2nd November. (35 days)
Only in exceptional circumstances should you ask a commercial client to pay in under 30 days. It is not considered professional unless it is a large new contract requiring a huge outlay for extra staff.
In the situation you are now in, you must be assertive. Any invoice that becomes more than 14 days overdue should be followed up by a Statement. A statement is basically the same as an invoice, but it is clearly marked as being overdue for payment.
Issue these statements every 14 days twice more (so initial invoice + 3 statements = 72 days) The final statement should state: "On (date) this invoice will become overdue by 90 days. If payment has not been recieved in full before this date, County Court proceedings will be initiated without further notice."
Then it is simply a matter of meaning what you say. Take them to court:
https://www.moneyclaim.gov.uk/csmco2/index.jspIt can always be withdrawn by a simple phone call, when they do eventually pay up.
Hope this helps.