I would say that you definately need to find out who that are acting for.(posted 8350 days ago)If it is the halifax then the Council of MOrtgage Lenders agreement and the Halifax's own statement means that they can no longer chase you for the money providing that it has been 6 years since repossession - if it is less than 6 years then this is irrelevant.
When writing to them to ask who thye are acting for - this should have been made clear in the first place - also ask for "a copy of the Money Order Judgement on which they are relying". They require this document in order to pursue the shortfall and if they do not have it thy must return to court to obtain one.
I would say that in the first instance that would be enough. See what they say and then come back to this site and let us know (e-mail me privately if you wish). If and when all the above is settled to your satisfaction thaen you should use the information on this site to ask for full documented proof of the debt and your liability. Take the time that you will have once you send your letter to read through the posting on this site - there are lots that are VERY useful - read the repossesion section as well.
Hope that helps but if you are confused do come back.