Sorry to hear about the problems.
Unfortunately unless your ex and yourself can negotiate your way to some form of agreed settlement, you may not have any choice but "to create an expensive legal case".
A word of warning - even with an agreed settlement, getting it written down, signed off, witnessed, executed and registered will not be free of legal costs, particularly with a mortgaged property at the centre of it. You'll have to include the mortgage company's requirements in any agreement in relation to the house.
I don't think messing with the mortgage repayments or even threatening to is a good suggestion as it may effect you and potentially your business.