Refund due in joint names 2146.77
Gross 8% per annum interest on refund 2527.47
Less basic tax to be withheld on 8% 505.49
Net refund due 4168.75
simple interest at a rate of 8% per annum which is to compensate the customer for being deprived of the money they had paid to the firm for the PPI.
This interest is taxable at source as explained in HMRC manual Ref SAIM2015 here
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
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.