VBNB36000 - VAT Business and Non-Business: Principles: Errors in Methods
Where HMRC has approved a method for a business we will have recorded the proposed workings of the method and will have shared this with the business. This helps to reduce the chance of misunderstanding as to how to apply the method correctly.
However, if a business does make a mistake when using an approved method, it can rework the calculation. This will be subject to the normal time limits for making adjustments. (See VR VAT Refunds Manual VR6700)