DWP denying receipt of letter

I sent a renewal form for pip, one year later I received a letter saying I was awarded nil points on every point. I asked for the assessors report to be sent, which took two weeks to arrive. Following this I applied for mandatory reconsideration in writing with a four page letter going through each point again. Today I received a phone call asking me if I still wanted to apply for a mandatory reconsideration !!! I said I had written two weeks ago but they are denying receiving it (sent signed for 1st class). I told the person I was unable to speak at the time and have sent another copy of the letter in the post today. They are calling me in the morning. I would rather they have my written letter before a decision is made I think we are all aware telephone conversations are not always recorded correctly. Can I ask them to ring me when they have found the letter or do I have to go through it all again verbally. I cannot believe they havnt got the letter or maybe they are just really thoughtful and thought I might want to appeal !!! Any help would be appreciated.

ALWAYS send letters of importance by recorded delivery. They have no excuse then.

Certificate of posting and keeping the receipt for that in regard of my letter to dWP re my misinterpretation of the “tell us once “ service when registering my fathers death in 2018 led to my sending a letter in jan 19 with my retaining the certificate.

I calculated (roughly) of how much of a certain benefit i had been overpaid and put the £ £ £ aside. Still waiting for them to come back to me. They cannot say that i did not inform them. I sent the letter and what they may have done with it (lost it) is not my responsibilty.

Nearly similar to what i did three years ago when i misinterpreted the meaning of my local councils “tell us once service” when i went to register my fathers death 4 years ago. Sent letter in jan 19 with certificate of posting and i have heard nothing since.

I did a rough calc of the amount and put the money aside.. if they do come back to me to say they have not received i have the proof and not my fault if the dwp know nothing at all