an instrument specifies for the purposes of property limited in trust a perpetuity period by reference to the lives of persons in being when the instrument takes effect,

the trustees believe that it is difficult or not reasonably practicable for them to ascertain whether the lives have ended and therefore whether the perpetuity period has ended, and

they execute a deed stating that they so believe and that subsection (2) is to apply to the instrument,

the instrument has effect as if it specified a perpetuity period of 100 years (and no other period);

the rule against perpetuities has effect as if the only perpetuity period applicable to the instrument were 100 years;

sections 6 to 11 of this Act are to be treated as if they applied (and always applied) in relation to the instrument;

sections 1 to 12 of the Perpetuities and Accumulations Act 1964 (c. 55) are to be treated as if they did not apply (and never applied) in relation to the instrument.