If the amount is understated or overstated in the return and additional tax is payable, or a repayment is due to the taxpayer, they must submit a further return or make a claim for repayment.
Os yw swm wedi’i danddatgan neu ei orddatgan yn y ffurflen a bod angen talu treth ychwanegol, neu fod angen ad-dalu’r trethdalwr, rhaid iddynt gyflwyno ffurflen arall neu wneud hawliad am ad-daliad.