If you are registered on the legal tile at the Land Registry as one of the registered proprietors then you have a joint interest in the property and it cannot be sold without your consent.
As the house is in negative equity were it to be sold now then there would be a shortfall for which you would be jointly liable to pay.
If the property market recovers and the house returns to positive equity then unless there is a declaration in trust you signed when you purchased the property you usually would be entitled to 50% of the proceeds of sale. Doubtless your ex-partner would dispute this on the strength of his mortgage payments and a court would certainly take this in to account were they to decide a fair division of the proceeds of sale (and your share would likely considerably reduced). You would be better off negotiating with him.
You could propose a transfer and remortgage to him, so that he takes over the mortgage and the property is transfered in to his sole name with him perhaps paying you a fee. Alternatively, you could agree a declaration of trust now stating the amount you each are to receive in the event the property is sold (with equity). You could also speak to a solicitor about an applying to the court for an order for sale once the house is in positive equity.
A final point - do you hold your interests as tenants in common or as joint tenants? If you hold as joint tenants I would advise severing the joint tenancy at the Land Registry, if you do not and you were to pass away he would automatically receive your interest in the property.
If this has been useful please kindly click accept so that I may be rewarded for my efforts It will be gratefully received and you will be free to ask follow-up questions.