Sockpuppet is correct, if you are registered for VAT then the invoice numbers need to form 'part of a unique and sequential series'. These are the full VAT Invoice requirements
- A unique and sequential identifying invoice number
- Your customer's name and address
- Your VAT registration number
- Date of supply to the customer
- A description sufficient to identify the supply of goods or services
- The quantity of the goods or services with a unit price - excluding VAT
- Name and address of your business
- The rate of VAT per item
- Amount owed without VAT added
It is self-limiting - anyone who cannot knock up an invoice in Excel with sequential numbering save and print macros probably won't be earning enough to register for VAT in the first place

from HMRC Frequently Asked Questions
Consecutive numbers on invoices Does the invoice number series have to be numerical, or can it include letters?
The ‘invoice number’ can be numerical, or it can be a combination of numbers and letters, as long as it forms part of a unique and sequential series.
In order to avoid the appearance of having only just started in business I do not want to start at Invoice no.1 and would prefer to commence with, say, Invoice no 1135. Is this allowed?
Yes, as long as the invoice series runs consecutively from that point.
I currently use a unique customer reference number as the identifying number on the invoice. If I add an additional consecutive number will this be acceptable?
Yes, this would have the affect of creating a unique and sequential series.
I have several customers and I wish to run several separate sequences of invoices at the same time. Is this possible?
Yes, it is acceptable to operate more than one sequence at the same time.
What if there is a break in sequence, for example where I cancel an invoice or it is spoiled and never issued to a customer?
As long as you retain the cancelled or spoiled invoice in your accounting records, or you can provide an explanation for the break in sequence, this is acceptable.

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