Thank you for that information.
As you have stated that you have remarried, you are now statute barred from applying to court for a financial remedy order in any case, so that ship has effectively sailed - you won't be able to go through that again even if you wanted to.
It's highly unusual that you have come off the deeds but remained on the mortgage.
I assume that the property is worth considerably more now, given the amount of work that has been carried out? Therefore the mortgage to value ratio will be much lower than previously, surely making a remortgage an attractive possibility. He really should get off interest only too, even if he has to extend the term to pay off the capital, as otherwise he simply faces a sale when he can't fund the capital lump sum. Any financial advisor should be telling him this, I would have thought.
The demand for £15k is not reasonable, you should not have to pay to come off the mortgage.
An application to court for a sale is a possibility, but it can be both very costly and lengthy to go down this route (outside of divorce proceedings that is) as it becomes a trusts law issue. Trusts law applications are one of the few remaining areas where acting as a litigant in person is nearly always a disaster I'm afraid, so you'd need a lawyer.
I would genuinely suggest mediation as a starting point. Your 'ask' is perfectly reasonable. You could, if you succeed in court, end up forcing him to sell the house. If he realises this and is told this from a recognised professional, I would be more confident in him coming to a reasonable conclusion.
I hope that this assists today?