Baffling back pay

I have just engaged a limited company client that has employed a number of individuals for a period of over three years. The business has been involved in various research and development projects and has not yet generated any turnover. The R & D costs have been substantially funded by an external private investor. It is alleged that the investor has not met his financial commitment and therefore the business has not been able to meet the payment of wages for over three years! The company is now involved in litigation against the investor for amongst other things unpaid wages. If the claim is successful employees will receive their back pay.
My question surrounds the PAYE and National Insurance treatment of back pay. If employees receive their back pay in one lump sum then a substantial proportion of the payment will be taxed at the higher rate. However...