any of the situations described in TCMA/1100 are attributable to a mistake in a return as to the basis on which the tax liability ought to have been calculated; and
ydy un o’r sefyllfaoedd a ddisgrifir yn DCRhT/1100 wedi digwydd oherwydd camgymeriad mewn ffurflen dreth o ran ar ba sail y dylid bod wedi cyfrifo’r rwymedigaeth dreth, ac