If you've not done so already, then read through the other postings and the information provided on this website. You'll soon realise that it is common practice that repossessed properties are sold for far less than they are worth.(posted 7851 days ago)You've got proof of the misselling from the advert in the paper. I hope that you kept it. If not, then get in touch with your local paper and get a copy - failing that your local town archives will probably have copies.
A person cannot be made bankrupt unless it is proved that you actually owe the money. These threats are usually made by solicitors who are banking on the ignorance of the person owing the money. Because you handed in the keys to your home, there wasn't a court hearing and as such there is no money judgement order. It is this order - confirming how much you owe which can then lead to bankruptcy proceedings.
The property was in joint names and so you and your ex are both liable for any shortfall and you should insist that the lender also contact your ex.
Solicitors are very persuasive with their threats, but do remember that they cannot force you to do anything, they're just very good at making you think that you have to do what they say. Write back to them and ask for a copy of the money judgement order - if there isn't one they can't bankrupt you and if you do find out that you were sued without your knowledge, then you can apply to have it set aside.
Ask the solicitors and keep asking them why the property was missold, they will have to go back to Northern Rock for the answers, so there'll be a delay. You also want to see a completion statement and a full breakdown of the charges making up the alleged debt. Again, look at this website for sample letters. Until Northern Rock's claim is fully justified, with invoices and receipts from third parties involved, then you do not admit liability for any of the debt.
I don't know if its illegal for a relative of an estate agent to purchase a property but if you think that there has been any dodgy dealing then you can get details of who owns a property from the Land Registry for your area. Check out the LR website for details. If you can make a connection between the owner and the estate agent or get other proof of a family connection, then you can if you wish start asking questions, but at the end of the day, the lender allowed the property to go for far less than it was worth.
Try your local CAB for legal advice and they'll be able to put you onto a solicitor best able to deal with your case. If there really isn't anyone in your area, then look to the next biggest town near you. There are lots of solicitors online and many of them deal with clients out of their local area, there will be someone who can help you.