PAYE54155 - Employer returns: regulation 80 determinations: onus of proof

HMRC bears the burden on demonstrating that the officer who made the determination thought that there may be tax payable under Regulation 67G or 68 that was not paid to HMRC nor certified.

Once HMRC has discharged its burden,  it is the employer's duty, on appeal, to show that the determination is wrong.

For extended time limit determinations, the onus is on HMRC to show that tax was carelessly or deliberately understated.

You may have to use the PAYE tax calculator, go to GOV.UK.