The Family Proceedings Rules 1991

Defendants

6.5  The defendants to an application under the Act shall be–

(a) the person alleged to have brought into the United Kingdom the child in respect of whom an application under the Hague Convention is made;

(b) the person with whom the child is alleged to be;

(c) any parent or guardian of the child who is within the United Kingdom and is not otherwise a party;

(d) the person in whose favour a decision relating to custody has been made if he is not otherwise a party; and

(e) any other person who appears to the court to have a sufficient interest in the welfare of the child.