State Pension Underpayment how long before payout from returning the form

I returned the relevant forms for my late father in law 4 weeks ago. I called to confirm receipt due to having bank details having to be entered and posted back on the form. However, I cannot get any indication or find anywhere!! that will tell me when the back payment will be received. I would call again (as should have asked) but last time I called I was waiting a long time and there may be others with more pressing matters waiting. I just do not like being in limbo and this came out of the blue anyway and was totally unexpected. An indication of it will be 4 weeks or 6 weeks etc is all I am asking for. Please can you give some sort of indication of how long it will take from receipt of the returned forms as like I said there is no indication anywhere of timescales. Many thanks

Hi,
Sorry, we can only answer HMRC questions on this forum.
Please contact the Pension Service for advice.
Thank you.

Sorry I thought you were one and the same gov.uk