We can only use the postcode on the tax return to establish the National Assembly constituency or Welsh Index of Multiple Deprivation areas in which a transaction took place.
Dim ond ar gyfer canfod yr etholaeth Cynulliad Cenedlaethol neu’r ardal Mynegai Amddifadedd Lluosog Cymru lle digwyddodd y trafodiad y gallwn ddefnyddio’r cod post ar y ffurflen dreth.