According to HMRC you should have the following:
You must clearly display the word 'Invoice' on the document.
You must also include:
- a unique identification number - (Invoice 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 the goods or service were provided (supply date)
- the date of the invoice
- the amount(s) being charged
- VAT amount if applicable - (only if you are VAT registered)
- the total amount owed
Sole Trader Invoices
- you must 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
Limited Company Invoices
- You must include the full company name as it appears on the certificate of incorporation.
- If you decide to put names of your directors on your invoices, you must include the names of all directors.
If you are VAT registered you must use VAT invoices, for more detailed information on VAT invoices click here - https://www.gov.uk/guidance/record-keeping-for-vat-notice-70021#vat-invoices
Free Invoice Template
If you aren't currently using an online or desktop accounting software package then why not use this free template (word document) and adapt it to your own company, details on how to adapt are with the download.