If I receive anything resembling a back-claim of tax, I will advise them that it is incorrect unless it also takes into account the tax I was required (incorrectly) to pay on the education grant element of my UAS pay whilst a student for 4 years... So, if they care to look at my records for 1969-1974, perhaps I may be able to assist further. If not, please $od off!!
Of course what doesn't help is that a lot of people haven't bothered with self-assessment and have relied upon the dubious skills of Binnsworth to make correct PAYE deductions. I always noticed a few corrections each year - usually in my favour.
Hector-the-revenue is normally pretty fair IF you've bothered to fill out his forms or on-line method. beacuse at the end of the day, if he doesn't believe in your honesty when you fill in the forms, then he isn't likely to be told anything again in the future!