Honestly, some people think that they can just walk away from their liabilities and that the piece of paper they signed called an agreement or contract is null and void as soon as they decide they no longer want to be bound by it - your ex seems to be one of these people.(posted 7461 days ago)Unless your ex was properly removed from the mortgage either by court order or by the Halifax and you agreeing with him to do so, then he is still bound by the deed under seal and can be pursued up to six years (voluntary CML agreement) from the default.
As to your own situation, you breached the terms of the offer accepted by the lender - therefore they have reneged on the deal and have come on to you for the full whack - I expect this will have been written into the agreement somewhere that they can do this if you failed to pay in accordance with the agreement where they accepted a lesser sum.
I suggest you invest in a decent solicitor to unravel this for you and that you tell the Halifax where your ex is and say that you will oppose further claims on yourself unless they do something about collecting his "moral" "half". If you don't do this, then they will come on you for the lot and regard you as the easier target and not bother with him.
Get some good legal advice quickly!!!