I would not worry about Black Horse, as you ought to get sufficient votes from other creditors to outvote any rejection from them - and in fact they may well support the IVA given that they are only owed a relatively small amount now.
Do you still own the property over which the NR loan was taken out for? If a good case could be put to Northern Rock, they may well accept an IVA. If you still own the property, how much equity do you have in it?
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