This should help.
36 Power of appointing new or additional trustees.
(1)Where a trustee, either original or substituted, and whether appointed by a court or otherwise, is dead, or remains out of the United Kingdom for more than twelve months, or desires to be discharged from all or any of the trusts or powers reposed in or conferred on him, or refuses or is unfit to act therein, or is incapable of acting therein, or is an infant, then, subject to the restrictions imposed by this Act on the number of trustees,—
(a)the person or persons nominated for the purpose of appointing new trustees by the instrument, if any, creating the trust; or
(b)if there is no such person, or no such person able and willing to act, then the surviving or continuing trustees or trustee for the time being, or the personal representatives of the last surviving or continuing trustee;may, by writing, appoint one or more other persons (whether or not being the persons exercising the power) to be a trustee or trustees in the place of the trustee so deceased remaining out of the United Kingdom, desiring to be discharged, refusing, or being unfit or being incapable, or being an infant, as aforesaid.