There are a number of things an invoice must contain to be valid.
Your government has some info on this.
Your invoice must include:
- a unique identification number
- your company name, address and contact information
- the company name and address of the customer you’re invoicing
- a clear description of what you’re charging for
- the date of the goods or service were provided (supply date)
- the date of the invoice
- the amount(s) being charged
- VAT amount if applicable
- the total amount owed
If you’re a sole trader, the invoice must also include:
- your name and any business name being used
- an address where any legal documents can be delivered to you if you are using a business name
If your invoice is formally wrong, i.e. missing some of the required information, your customer can rightfully demand to get a correct invoice from you before paying anything. After all they would get in trouble with the tax authorities for accepting an incorrect invoice.
If your invoice is formally correct, then it is valid and should not be changed. In fact, a valid invoice should never be chanced. The only way to correct an invoice that is formally correct is by issuing a credit note canceling out the value of the invoice.
If your client does not accept a formally correct invoice after some discussion, I´d send them a payment reminder and if the still don´t pay I´d threaten to give the matter to a lawyer or collections firm.