How to reply to HMRC letter regarding my claim for tax relief

HI, I received a letter from HMRC with regards to my tax relief application. The letter asks for certain evidence like receipts. How do I reply? There is no email or site to upload things or questions. I tried calling HMRC, but they always say they are busy and the line just cuts off. I feel if HMRC asks for evidence, then there should be an easy way of replying.

Hi Whysodifficult,
If asked for evidence you would need to submit the details in writing.
The address would be:
Pay As You Earn and Self Assessment,
HM Revenue and Customs,
BX9 1AS UK.
Thank you.