Ex won't finalise mortgage takeover?

My partner has been divorced for 3 years, his ex has moved out well before then.
They own a house purchased for 85k and currently sitting at 70k and been surveyed to sell at 60-60k so in negative equity.

We wish to take over the house, at first she was ok with it, she signed the lender document from the mortgage company very quickly.
Now we require the land deeds to be signed from the solicitors to officially own the house, we have done our part but she had now changed her mind it seems and not signing.

We are paying her legal fees to do this and been paying the mortgage all this time without her paying a penny towards mortgage or repairs.
She's made several comments asking if the house is worth money and we have been honest and provided details showing it is not.

Is there anything at all we can do to remove her since she isn't paying for anything? Sadly it is just a diy divorce and text agreements that when the time comes to remove her name when he could.
We don't expect her to pay towards the house since she doesn't live here but would prefer to own it since we are paying for it!

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