You should have declared the part ownership of the house, and the secured debt (i.e. the mortgage) on your bankruptcy form. The Official Receiver should then have investigated what (if any) your interest was, because he/she could demand that the house be sold and your share of the equity go to your creditors. If you did this, then any liability you might have - now or in the future - to the mortgage lender is included in the bankruptcy and you would not have to pay anything to the lender.
Similarly, your ex should have declared his interest in the house, and the same will apply to him.
If you did not declare your interest, then I suggest you contact your Official Receiver and do so. You will have to explain why you did not declare it. I think (but am not certain) that the Official Receiver has powers to re-open your case so that your interest in the property, and the mortgage, can be included. If that is not possible, then it could be that the mortgage lender could look to you for any shortfall (i.e. if the house sells for less than the outstanding mortgage + costs after it is repossessed, or after an OR initiated sale).
If there is not much equity in the property the OR will ask your ex (& you, in relation to your interest) whether you can arrange for a third party to buy out his/your interest. If this is done, then your ex can go on living there, & you remain jointly responsible for the mortgage. If that happens, I suggest it would be vital for you to get removed from the title deeds and mortgage as soon as possible.