unexpected payment from HMRC SA

I have recieved a largee(ish) payment from HMRC SA. This payment is not in line with my self assessment calculation and does not appear on account. Where should i report this to. i cannot get through on the helpline

Hi,

Our helpline is very busy at the moment. If you can contact us earlier in the day it is less busy.

Self Assessment: general enquiries

Thank you.