To ensure you're not liable for penalties, make sure you issue and receive watertight tax invoices that adhere to these requirements.
Eight requirements of a valid tax invoice
Requirement#1: The words 'Tax Invoice', 'Credit Note' or 'Debit Note', depending on the document required.
Requirement#3: The name, address and Vat number of the purchaser. If you sell to someone that isn't a vendor, leave it open;
Requirement#4: A serial number of the invoice;
Requirement#5: The date the product/service was issued;
Requirement#6: A description and the mass or quantity of the goods bought;
Requirement#7: The amount paid for the goods bought; and
Requirement#8: The price of the goods, the amount of Vat and the total, or one amount including the Vat, but then a statement that Vat is included in the price and the rate at which it was charged (i.e. either 0% if zero rated, or 14% if standard-rated must appear on the tax invoice).
Ensure you issue and receive tax invoices that stick to these requirements.