PART XITIME OF SUPPLY AND TIME OF ACQUISITION
Royalties and similar payments91.
Where the whole amount of the consideration for a supply of services was not ascertainable at the time when the services were performed and subsequently the use of the benefit of those services by a person other than the supplier gives rise to any payment of consideration for that supply which is—
(a)
in whole or in part determined or payable periodically or from time to time or at the end of any period,
(b)
additional to the amount, if any, already payable for the supply, and
(c)
not a payment to which regulation 90 applies,
a further supply shall be treated as taking place each time that a payment in respect of the use of the benefit of those services is received by the supplier or a VAT invoice is issued by the supplier, whichever is the earlier.