Thanks for your question
The PPi repayment itself would not have suffered incokme tax, but any payments if interest made would have.
I would assume you have been provided with documentation detailing the PPI repayment the interest and tax suffered, and a a non taxpayer you will be entitled to a tax refund.
You should write to HMRc with your P60 (for any private pension) details of any state pension and the document detailing the tax suffered on the interest payment after 05/04/2015 - HMRC can then make a tax refund to you (keep copies of the letter and the PP1 documentation)
Make sure you provide your full name and address and National Insurance on the letter.
Let me know if you need an address to write to (should be on your ;atest P2 notice of cosing, but if you cannot locate this OR did not get issued with one, then let me know)
Also let me know if you have any follow up questions on the infroamtion I have provided.