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You have stated that you are a non taxpayer. If you think you are entitled to a repayment , you should complete Form R40 - Claim for repayment of tax deducted from savings and investments.You can download the form from HMRC website here https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/419506/R40_M__internet_v2.0.pdf
More information on this can be found here
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