Oh for heaven's sake!
Stop saying £4K isn't much money - it's not about £4K, it's about him getting off the mortgage which can only happen if she can take it over (v v v unlikely it sounds) or the place is sold and he can cover the full shortfall - because he'll be liable for the full amount jointly, and she won't pay it, possibly.
But... why on earth did you marry a man who hadn't sorted this out? You didn't have to marry. You knew it was outstanding.
I would love to hear the other side about the "bellend" who left her. I'm just going to throw out a guess that he wasn't exactly 50:50 carer, huh?
The XW may be a bitch.
She may be a woman with MH problems (actually she sounds like it) who is scared of losing her home and stalling.
She could be both of course.
But the time to sort this out was when they split up - you are in this position from his stupidity as much as her stalling, be that from desperation or full Machiavellian plotting 
It is ridiculous that OP is saying that XW can always go live in her boyfriend's house.
Shall we take a moment to imagine that post from XW in Relationships?
Come on, you know the reply - do not leave a secure home when you are low income and unwell, for someone who is just a boyfriend.
You can simply be sensible without being being a grand schemer.
It would do my head in OP, really it would. But it's my husband in your case that would have me tearing my hair out!
And btw I'm a single mother who managed the financial settlement herself in divorce and successfully tied up all loose ends. That you did too is totally irrelevant.
Perhaps we weren't blindsided by being dumped and left with young children by a bellend and had the mental capacity to cope?
Perhaps we weren't hurt so badly we never felt vengeful?
Perhaps we were simply a bit smarter, or a bit more confident with legal terms, a bit more willing to let go...
Crazy cat lady doesn't sound like she planned all this so much as ran from it. Which he also did, in his own way. Two ostriches in a bucket of fucked up sand.
In his place, I'd recognise that she was in a vulnerable position and also that she doesn't have any hope of getting a remortgage, that an order to force a sale would be long and costly and that he could lose more to her (pension) because he stuck his head in the sand JUST LIKE HER.
I would find out exactly what the endowment is worth (stock market has been rising for 2 years remember) and how much is owing on the mortgage. It's all in joint names, he doesn't need her for that.
Then I'd look at the possibility to pay the mortgage off, retaining a charge over the property in return for doing so. Repayable on marriage, death, whatever... That leaves him clear to get a mortgage with you.
If he has no capital I'd look at him remortgaging (after cashing in endowment, if it has no bonuses to come) because rates are low and can be fixed, and the LTV will be excellent. There is a possibility that he can have two mortgages if his income is sufficient.
But really - stop bitching about her. Even if she's totally astute, a bitch and cackling to her cats... he put himself in this position!