stamp duty under the heading “conveyance or transfer on sale” in Schedule 1 to the [1891 c. 39.] Stamp Act 1891 is chargeable on an instrument to which this section applies, and

part of the property concerned consists of exempt property.

the consideration in respect of which duty would be charged (apart from this section) shall be apportioned, on such basis as is just and reasonable, as between the part of the property which consists of exempt property and the part which does not, and

the instrument shall be charged only in respect of the consideration attributed to such of the property as is not exempt property.

an instrument executed in pursuance of a contract made on or after the abolition day;

an instrument which is not executed in pursuance of a contract and is executed on or after the abolition day.