Open Question: How to perform NAV calculation - Private Equity Funds?

I know the basics - equity company and investors go in together in a fund. That fund is used to invest in companies, and the private equity firm will have many separate funds going at the same time. I believe that each fund can be used to invest in 1 or more companies. I also know that each day the performance of a fund is calculated using the Net Asset Value (NAV) which is basically the surplus left for investors after deducting all liabilities from assets. Now - how is NAV calculated? Do the fund accountants have access to the records of the company(s) and calculate a basic balance sheet/financial statements everyday? Or are references made to fair value? Thanks for your help James (This is based on UK principles - I don't know if US is different)