Go to the Section "Repossession" on this site first of all and read the Do's and Dont's. Also read the info on SARN's (Subject Access Request Notice.)(posted 7880 days ago)You will need to serve a SARN on the Halifax and you'll find a copy of a standard letter in this same section. The Halifax will probably write back telling you the cost for providing the SARN info is £10. SEnd them a postal order (the least info they have on you the better) Send it Recorded Delivery and keep the slip.
I would also perhaps add a bit to the SARN letter saying "I refer to recent communication etc and have to advise that I cannot at present accept any liability for the monies you are demanding. Perhaps when you provide the relevant documentation in support of your claim, I may be in a position to review my position"
Because you've been paying some money it might be chancing it a bit but you've nothing to lose I think.
They must reply to your SARN letter within 40 working days. If they don't reply come back to this site and you'll find out how to complain to the Information Commissioner.
Don't panic...don't phone them or talk to them on the phone.
I'm in the same position as you and all I try to do is keep reading the info on the site. It can be a bit mind boggling at first but eventually it will sink in!
If you have more queries e mail me or put a posting on the site as there are others who know millions more than me.
Hope this helps
Moira