VAT adjusting invoices after payments

eddy2309

My client failed to register for VAT and exceeded threshold August 2014 so we now have to work out what should of been charged etc. Some of his work is CIS deducted. Could anyone advise me on the correct way to re-do the accounts to allow for VAT. All customers and contractors have paid and so unable to re invoice them. For example;
Original invoice: labour £1,800 materials £3,000 less 20% tax deducted £360 = £4,440 paid.
Thank you.