Postal address

[CYNthesys]

Son recently divorced, jointly owned marital home on market for sale, ex has an occupation order allowing her to remain in home until property sells. CAO in place whereby son currently has the children approx 40% of the time . Son has no fixed abode until such time that the house is sold so his post continues to be sent to former marital home and ex sends his post with the children. Ex now demands that he has post redirected to a relatives address , failing which she will write return to sender. As he is struggling financially Royal Mail redirection fees are unaffordable at the moment. As joint owner of the property is he within his rights to continue using former marital home as his postal address?