Thanks for your question.
It's an exaggeration for him to say that you have "no choice" but to pick one. It would be more accurate to say that there will be consequences if you do not pick either. This will probably be in the form of a suggestion from his solicitors that he will apply to Court for an Order for sale of the property from Court. He would wish to avoid this if possible as he will incur legal fees.
The liability under mortgages is joint and several which means that the lender can sue either or both of you if there is any debt, regardless of any understanding you had with your partner about what payments would be made by whom. This means that it is crucial that the repayments are made otherwise your credit record would be affected and you would both find it difficult to raise credit in the future.
If you simply state that you will only pay what you have been paying earlier then you would in effect be paying a game of brinkmanship hoping that he flinches first and continues to pay at the rate he currently pays at. This is risky, but if there was an agreement that between you to that he would pay more towards the mortgage repayments then you could go back to them that you consider this to be a legally binding agreement between the two of you and see what they come back with (they will deny undoubtedly) but it will at lease by you some time.
I would attempt to string it out for as long as you can (ie. be slowly cooperative) and keep a very close eye on the mortgage account to see that the repayments are made.
Ultimately though, he can apply for an order for sale, but once he has applied it can take a number of months to actually reach the stage where an order for sale is actually made so don't feel that you have to dance to his tune, but make sure the repayments are made (regardless of who pays what.
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