Death of a sole Trustee

Sole Trustee (“X”) dies.
X’s son (“Y”) and X’s daughter (“Z”) are X’s personal representatives.
By virtue of section 36(1)(b) Trustee Act 1925, Y and Z have the power to appoint Trustees following X’s death. Y and Z do not exercise that power.
Y subsequently dies.

Can Z solely appoint a Trustee under s.36(1)(b) or would Y’s personal representatives have to be party to the appointment too?

I appreciate the answer may depend on the terms of the Trust instrument.