Student Finance National Insurance Number

Hello, I have just gotten my national insurance number to update my student finance account. For some reason, it says the number is already in their system so I have to send them evidence. They say I need to post my original document, which I have, but I am unsure of where to post it, as there was not much information given, such as an address. I want to make sure I don't send ot to the wrong place so I don't lose my documents or lengthen this process any longer. Any help would be appreciated!