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.
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