I am just back from 3 weeks in the rain. This is to let you know, I (& many others) are likely owed many thousands; I stand to recoup over 5K UKP. I know next to nothing about Income tax coding which is at the crux of the scam. Each year I am given a notice of coding & accepted it unconditionally as it was from the HMRC people themselves. This is their defence when challenged; if you accept it without checking, you lose.
My daughter thought I was paying too much. We checked on line & found (eventually) to my horror that the tax authorities have assumed I was receiving a full State pension (of 10K this year.) In fact my pension has been frozen for 15 years as I live in Thailand.
I sent two letters from Thailand which were ignored. I sent on-line messages which were allocated an official case number but never acted on.
So I phoned in the UK. (Phoning from here was difficult because of the time gap + an inordinate wait of about an hour before you got a real person (all the while being told how important your call was). I was initially re-directed to the Pension people in Newcastle, staffed by incompetents who did not know what day it was & re-directed me back to the Tax people.
After an exhaustive conversation with someone prepared to go the extra mile, he admitted that I have been overpaying for years. He could only go back 4 or 5 years and calculated I was due 5K+. I received the 1st tranche within a week (3,500+) but there has been a deafening silence since.