Hi Shani,(posted 7267 days ago)First of all don't talk to anyone on the phone or meet with them.
Now, regarding the SARN. Halifax do not need your ex-partner's signature as although this was a joint mortgage you are asking for it in relevance to yourself.
If they are chasing you for a shortfall then they are well aware what this is in connection back. Do not admit the debt, always refer to it as an alleged debt.
Write back to the Halifax stating that you want all relevant information regarding the account (give them your mortgage account number) and that if they do not respond to you with this information you will have no option but to complain on an official basis.
Advise all parties that you no longer, due to the seriousness of this matter, wish to discuss this over the telephone and that ALL communication must be done by writing.
Don't be scared of the SARN. This is your right, under the Data Protection Act, to have access to all information they hold about you (although if you are going through a solicitor some of their stuff is subject to legal privilege).
Don't meet the debt collector, of course he's nice he wants your money. The onus is on the Halifax to prove you owe them this money and that is all you are asking.
Good luck.