This is what you find when you go to the "Contact" part of their website (address:
http://www.studentfinanceni.co.uk/portal/page?_pageid=54,1267813&_dad=portal&_sc...)
"If you want to talk to someone about Student Support please call the
Student Finance ni Customer Support Office
Phone 0845 600 0662
Minicom 0845 604 4434
(You must have access to a Minicom device in order to use this service).
Opening hours are:
Monday to Friday, 8.00 am to 8.00 pm
Saturday and Sunday, 9.00 am to 5.30 pm"
No sign of a postal address or of a geographical fax number, unfortunately.