Severance — s.36(2) written notice; service

Three friends — Anya, Ben and Carl — buy a house together as equitable joint tenants. Relations between them sour. Anya writes a letter to Ben and Carl stating: 'I am hereby severing my joint tenancy in the house with immediate effect and I wish to hold my share separately from now on.' She signs and dates it and posts it by ordinary first-class post to the house, where all three live. Ben reads it. Carl, who is away, never sees the letter because Anya later retrieves it from the doormat and destroys it before he returns. Which of the following best describes the effect of Anya's letter?