Re very late vat registration

One more question.

We are going to try to re-invoice the customers with the vat amount to recover the vat amount that should have been charged. If the customers do not co-operate, will the vat liability be calculated as 7/47 (17.5%) or 3/23 (15%) of money already recieved or will it be 17.5% or 15% of money already recieved.

Thanks again

Claire