"Can anyone confirm if its possible to do this without involving the mortgage lender."
Not possible.
Its real simple. You and your joint owner own the property jointly and you have a joint mortgage. You are both jointly responsible.
If you can remortgage the property with anther lender and get a loan for the full amount and discharge the current mortgage then you can transfer the property into your sole name.
What does you joint owner say about their joint responsibility. Can they pay off some of the debt? And if not, why do they think they can walk away from their debt?
Not wanting to continue to pay their joint debt is not any kind of answer.
mf