Idem servicing debt issue - complicated

Hello everyone,
I've used the forums in the past but haven't been on for some time due to the stuff of life and also seemingly not having many issues with debt recently. However, things return to haunt us if not sorted properly so here's my situation. Sorry if it's long winded but i'm giving as much info as possible in the hope that someone can advise me.
I have a joint debt with the ex husband which was originally a secured Picture loan, then sold to Webb and now with Idem servicing who I believe are a DCA? In February 2012 the debt became unsecured when Webb agreed that if we sold the house, paying off the mortgage and partially settling the secured loan, they would remove the charge to unsecured on the understanding that if either of us took ownership of another home, the charge would go on to that property for the outstanding amount. In the meantime agreed payments were made on the now unsecured loan. We never bought another house and divorced in April 2017.
As the loan could not be paid off I wasn't able to get a Clean Break Order from the ex leaving us both severally liable. I asked that idem split the debt between us but they refused saying they would consider a settlement offer from either of us! Payments were made to the debt until January 2016 after which payments ceased due to the ongoing divorce proceedings and because a way forward could not be agreed.
So I now find myself trying to clean up my debt/credit with a continuing financial connection to the ex who is now remarried. The debt also remains on my credit files at 28k. No payments have been made for more than 2 years and I have not heard from Idem since asking them to split the debt, my last correspondence being in December 2016. On my credit file it says the loan started in 2007 which is when it was taken out with Picture. It shows no defaults although this sort of did happen as a payment arrangement was entered into with Webb and then Idem. The file just shows payments as late and on some credit files it shows as still being secured!

I live with my new partner in his house which he owns outright but I have no stake or claim on.
I'm wondering what I can do to get it removed from my file - why hasn't it dropped off? I've placed notice of correction requests with the referencing agencies to say it's an old debt and that it isn't secured but Idem refuse to budge. I think it's likely that there is no original CCA making it unenforceable. While I'm happy not to have heard from them I don't want to suddenly find they're taking me to court etc.
The ex has disappeared to Spain so they won't find him and I'm trying to start afresh with someone who has never had problem debts. My partner says is it worth stirring things up if I haven't heard but my point is it's still sitting there. I can't pay it off, not even half of it as a) I don't see why I should take full responsibility and 2) i can't afford it or get another loan for partial settlement as my credit score is still dire despite me managing my credit cards and short term loans well in recent years. I now have only small balances on a couple of credit cards and have cleared a small bank loan all of which should help my rating if it weren't for this huge 28k!
So I'm at a loss at to what to do to move forward. Do I request a CCA to see if it's enforceable? Then at least I'd know if it could go to court or not.

Please help! My divorce solicitor was no use as the debt is joint and couldn't be settled. I want this behind me.

Thank you for any support, advice or guidance