I am not entirely sure about this, but if the taxpayer has supplied the required information then any mistake by HMRC is not something they can later visit on the taxpayer unless they can show that the information supplied was in fact not only incorrect but that the TP knew it to be incorrect.HMRC said I had been overpaid tax credits & I had to pay it back. I didn't have hundreds sitting around obvs lol .... er hello im on tax credits for a reason doh & it ain't cos im loaded !! So I told them I had given them the req'd info & if they are unable to work it out correctly that really isn't my fault or problem
However, it is not unusual for some eager clerk to try it on anyway. HMRC are prone to fishing expeditions in cases where there is no actual evidence of wrong doing or underpayment but circumstances just seem to show there might be something. The TP is then left with the task of proving a negative, and demonstrating their innocence. I further suspect that few people have sufficient confidence in their position to tell the HMRC to prove their case or go away.
Usually you need to hire a tax lawyer, at what can be considerable expense, to check out the facts and then tell the HMRC to put up or shut up.
Whatever the case quoted above, it will not effect the ultimate pension entitlement, as this is an entirely different matter which is based on National Insurance contributions made or credited over many years.
The quoted TP would be well advised not to send any pointless notices, poking the bear results really depend on how deeply asleep he is.