I applied to bank to take over mortgage in my own name but they refused. Now the joint account is still in being and I just pay full mortgage on my own. The interest in the house has been transferred into my name.
Are you sure about this?
There are two things
Ownership of the house.
Liability for the mortgage.
If UB refused to allow your ex off the mortgage, it's very unlikely that his name is off the deeds. You and he may have made a side agreement between you, but in reality, if you don't pay the mortgage, they can pursue him. It might be theoretical if he is in Australia.
From your point of view, if you pay down the mortgage and eventually return to positive equity, if his name is still on the deeds, you won't be able to sell it without his approval. He may insist that he owns half the house and you will spend a lot in legal fees defending your position.
Assuming he is still on the deeds, you should not pay any more than interest only.
By paying the full repayment, you are paying down your ex's share of the negative equity.
It seems to me that your mortgage is unsustainable.
You need to have this conversation with UB and see what deal they offer you.
They do very few split mortgages, but they might offer you an interest rate reduction for 5 years.
If you can't reach a generous agreement, go bankrupt
Tell UB that you intend to go bankrupt. They might do a deal with you to allow you to sell the house in an orderly manner and do a deal on the mortgage shortfall.
Don't bother with trying to rent out the house
You need finality on this. You made a mistake when you were 21. You now need to draw a line under this. Don't mess around with "solutions" which will leave you insolvent for the rest of your life.