An agreement in principle to sever a joint tenancy is not enough to amount to a mutual agreement or a course of conduct.
In Gore and Snell v Carpenter ((1990) 60 P & CR 456) a husband and wife owned two houses as joint tenants. They agreed in principle that they would have one of the houses each (and end the joint tenancies). A draft separation agreement included a clause severing one of the joint tenancies but this agreement was part of a package of proposals. Final agreement on the proposals had not been reached by the time of the husband’s death. Judge Blackett-Ord held that there had been no severance. It is a question of intention in each case (at 464). In this case, the parties had come close to agreeing but had not agreed. There was no course of conduct. This requires both parties to commit themselves to a severance but the wife never had.