OK - so they made a mistake, but so did you when you signed your proposals which presumably did not include the creditor. At the end of the day the onus is on you to make sure that the documents which had been prepared for you were accurate. Did you check the documents before you signed them?
I am sure that Northern Rock will agree to join in your IVA, and your IP will need to address how to deal with any dividend issues which may arise from the inclusion of an additional creditor.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
To have me propose an IVA for you, please visit:
http://www.melaniegiles.com/ivaEnquiry.asp
See customer feedback at:
http://www.iva.com/iva_companies/IVA_Advice_Bureau.asp