Invoice requirements if payments received in advance
If you receive payment in full or in part before the completion of the supply of goods or services, you must issue a Value-Added Tax (VAT) invoice in respect of each payment received. The VAT invoice must be issued not later than the 15th day of the month following that during which each such payment was received. This does not apply in the case of intra-Community supplies of goods.
There are special rules for deposits.