just ask them to provide copies of all bills, and payment dates to give you an approximate idea, then just wander into your bank and ask them to look up payment amounts of…etc.
Alternatively just tell them to **** off, they’re the ones that have buggered it up. Ask them what reconciliations they have, as you know you have paid, so there should be credit balances in their system which they haven’t reconciled correctly. Then ask them about their accounting policies, and whether they, on the basis of what you’ve said, believe that their accounting policies are correctly disclosed.
Then tell them to **** off again, and to stop wasting your time, and that any time undertaken by you will be billed at an hourly rate of £48 per hour, commensurate with your employment rate.