OK. You will have to change joint tenant to tenant in common. This is a very simple and inexpensive process so phone your local District Land Registry Office who will guide you through and tell you their fee. The consent of your ex partner is not required. As a tenant in common you can then apply to the County Court for an Order to sell stating that the whereabouts of the other tenant in common is unknown. You will eventually get the Order but the Court will require much reassuring and convincing beforehand that the other tenant is truly "lost". Without the Order the sale will be blocked at the Land Registry and they will not budge until you get the Order.