Additional information for Home Responsibilites Protection application

I have received a letter today concerning my HRP claim, telling me I needed additional information for my HRP claim. I logged in to my account, and it would not let me edit or add information to my claim as it had already been completed. Can someone direct me towards both a phone number and a mailing address, please. I'm asking for a mailing address also because I know the wait times can be quite hefty, and I am pretty sure it charges per minute via my phone service. Much thanks.

Hi LStuart71,
The relevant telephone number is 0300 200 3500 (National Insurance helpline). However, I recommend that you post the additional information requested to Decision Making Team, PT Operations North East England, HMRC, BX9 1AN. 
Thank you.