Given the continual (it appears) reports of people being given wrong codes, paying less than required and demands for payment, you may be wise to put your windfall in a safe place - just in case.
I am in much the same position as you - a pensioner with fixed income - and have been very wary of such good luck ever since I received some eight letters from HMRC in one week advising that I had various codes, had underpaid and/or overpaid and was to be refunded or expected to pay various amounts. It took some time to sort it out and in the end nothing changed from before the letters arrived. Apparently a computer error - nothing to do with human beings then!!
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