Tax overpaid

Hi, last week my employer (agency) overpaid me Ł115, they sent me money that suppose to go to other worker, they called me and demand to return money and so i did, i give Ł115 to that worker yet i overpaid Ł35 deductions (Ł20 tax Ł15 NI), they ensured me that i will pay less/wont pay tax
for this week but they took money for tax/NI and when i call them they told me they cant do anything about it, what should i do now?

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