Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question.
My gut reaction is that you are being fobbed off by HMRC in the telephone call. The law in this only goes back over a century and is the rule in Bwllfa and Merthyr Dare Steam Collieries (1891) Ltd. v Pontypridd Waterworks Co.  AC 426!
I would suggest that you confine your negotiations with HMRC to the written word. Write to them and point out that the payment received was for an earlier tax year and accordingly you are seeking to revise your tax position for that year. They will, naturally, not like it as they will loose revenue, but we were not put on this earth just to fill HMRC's coffers! If they argue ask them for the statutory authority upon which they base their ruling, NOT their manuals which frequently state the law as they would like it to be, not what it actually is!
I do hope that I have shown you a way forward in this matter.
Thanks! As an ex revenue officer myself I thought I was on the right track but the attitude but have waited since April for an answer! MMmm, methinks you are right....Thanks, ***** ***** proceed onwards.