Only way forward it seems from a bit of quick research is to apply to have the unilateral notice removed on the grounds that you contest the liability or that it is out of time (i.e. statute barred under the mortgage voluntary code) if that is the case.(posted 7206 days ago)You could then sell the property without Northern Rock taking the remaining equity in payment of their alleged dues without going through a court order.
I assume that there is not already a court order for this debt - if there is, then it changes my answer in that the statute barred bit does not apply.