Tax charged under section 126 above on an amount determined under section 127 above shall be charged at the rate or rates at which it would have been charged on the death first mentioned in section 126 if—
that amount, and any amount on which tax was previously charged under section 126 in relation to that death, had been included in the value transferred on death, and
the amount on which the tax is charged had formed the highest part of that value.
In determining for the purposes of subsection (1) the rate or rates at which tax would have been charged on the amount determined under section 127, the effect of Schedule 1A (if it would have applied) is to be disregarded.