Readers’ forum: Issuing VAT invoices after registration number arrives

One of my clients waited a long time to obtain her VAT number from HMRC. For this reason she issued invoices that charged VAT in the meantime – however she did not show the VAT separately in her invoices. In other words she would issue an invoice of £120 including VAT rather than £100 plus VAT.

At long last the VAT number has arrived and my client needs to issue VAT invoices to enable her customers to claim input tax on the retrospective invoices.

How should she go about this? I wondered whether my client should issue a credit note to cancel the original invoices ie a credit note for £120 including VAT and a new invoice for £100 plus VAT?

If so should both documents be backdated because the customers would – I guess – have claimed input tax according to the original date?...