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HMRC internal manual

Company Taxation Manual

Corporation Tax self-assessment (CTSA): the payment obligation: repayment of IT

The claim for repayment of income tax (IT) must be made in the company’s return or in an amended return.  (FA98/SCH18/PARA9 and PARA57)  

The return form does not call for vouchers in support of claims to repayment of IT.  You should not ask for vouchers otherwise than in the course of an enquiry into the return.