mdog69

You may also want to consider raising a complaint to the bank, as "proving a negative" is very difficult. (For example I can easily prove that I have a bank account with X by producing a bank card, statement, or other correspondence, but how would I prove that I don't have a bank account with X - the answer is "I can't"). Note also that it is likely that any initial call to the bank would be to someone who probably knows nothing more than "Computer says prove you are not a UK tax resident", and you will probably have to get the matter escalated.

Short answer - they wouldn't be worse off. Longer answer: It doesn't work like that - you pay tax AND national insurance at the appropriate rates on earned income. Once you reach state pension age you only pay tax on earned income. Note I've deliberately used the term "reach state pension age" rather than "becoming a pensioner". It's your age that is the trigger to stop paying NI, not whether you are receiving a pension.

Does she know a colleague who worked for the company at the same time? If yes, ask the colleague if they can find a P60 or a P45 - this will have the employer's "PAYE reference" (usually in the form of 123/ABC456 or 123 ABC456). This may provide further information to link your mother in law to her NI number.

From my experience three years ago (waiting to see what happened for 2019/20), it took some time for the DWP to tell the NI people which credits have been applied, which in turn changes the pension forecast data. You might expect the transfer to happen around May (when P60U gets sent out), but it happens some months later. If you are not retiring this year, it's a case of sit back and wait then when it happens, it happens.