Adding invoices outside of financial year

Can anyone help?

I have been sent a list of inquiries from my client’s accountant.
One of the problems is when we started the accounts we did a lot of bank tagging and creating invoices for the items, which we then should have located the invoices for. This didn’t always happen and I tried to get round this by creating payments on account.
Unfortunately we have left payments on account and now I’m unsure if I can create invoices for them.
My client is vat registered (flat rate scheme) and the financial year runs from 31.07 to 01.08
Although I may be able to locate some of the invoices I don’t know if I can put them on the system.
I’m not experienced with vat but gather that as long as I don’t effect the turnover figures I can ammend the details…The more recent invoices are one thing and all payments have been tagged. The accountant has flagged up prepayments from 2014…The accountant does not advise on solutions. It would be nice to get the Purchase Ledger straight.