Ripened residence

My client owns a piece of land that is ripe for residential development.

 A self-employed planning consultant agreed to apply on our client's behalf for outline planning permission on the land. In recognition of this my client agreed to pay the consultant a fee based on a percentage of the sale price of the land — and payable after the land was sold — if the application proved successful

Planning permission was eventually obtained and the land was sold eighteen months later. During the eighteen months the planning consultant was obliged to register for VAT.

My question is whether VAT is due on the fee payable for services rendered and completed before VAT registration given that an invoice could not be raised until after the sale of the land by which time the service provider was VAT registered.

Would a solution have been to issue an invoice...