They wont pay you

I was contacted by Willie Sewpersad who needed urgent help as they were potentially about to lose their ISO certification, having not responded to audit findings and not provided a response plan. We agreed verbally on a price and I went ahead and compiled a response plan template and inserted some possible responses. I arrived on time as scheduled and began working on the response in Willies office and after a few hours he suggested we transfer the plan to his computer and finish it together. It was copied across from my PC to his and we sat together at his computer and finished the response to his satisfaction. Where he saved it on his machine, I have no idea. I left later that afternoon at which time he was satisfied with the work undertaken. I was contacted by him at a later time as he could not find where he had saved the work. I suggested that he check his MS Word recent docs and offered in fact to retype the entire response if he could not find it. I heard nothing and assumed he had found it, until I had submitted my invoice and had been querying the payments for some weeks, with no response from Willie. I called the office and obtained the email addresses of their Financial director Akshir Heeralall and one Tsakane (the CEO I was advised) and copied them into my emails. I got a response from Akshir requiring that I provide evidence of having performed the work. I explained that Willie had it on his computer. They have refused to pay unless I can provide the document which I am of course unable to do as it was done on their computer at their request. I offered to have an independent IT specialist do a search on the computer, which was refused. Don't work for them without a full upfront payment.