If the above options do not fit a particular business then HMRC does allow what it calls a ‘special’ method to be used. A special method is any calculation, other than the standard method, that enables the business to calculate how much of the input tax is recoverable. This method is unique to the particular business circumstances.
Written approval is needed from HMRC before it is used but it may be a more convenient way to calculate inputs where the established methods produce results which are not fair and reasonable.