Finding deceased person's NI number

I contacted HMRC to attempt to find out the NI number of my late mother in August however I haven't received a response. I sent the letter with all of the information that the call centre agent told me to send. How long should I expect to wait to hear from them?

Hi Andrea,
The Team that will be able to help you are currently dealing with queries received in February 2024 so, unfortunately, there will be a delay before you receive a reply.  
Sorry for any inconvenience.
Thank you.

Hi Andrea Who did you contact? I am an executor of a will and need a deceased's NI. The person died over 10 years ago. I seem to be going round in circles and getting no-where trying to find a contact number or email to ask for help. Do you have any idea where I can start?

I contacted the NI helpline and they advised me to send a letter with all of the deceased person's information in - full name, date of birth, date of death, address. I also enclosed a copy of the death certificate and my own ID. I haven't heard anything back yet but as you can see above there seems to be quite a backlog. Hope you get sorted soon!