£100 immediately, whether or not you owe any tax, and it escalates from there. After three months, daily penalties of £10 accrue up to £900. At six months, a further penalty of 5% of the tax due or £300, whichever is greater; the same again at twelve months. Late payment penalties are separate — 5% of the unpaid tax at 30 days, six months and twelve months — with interest running throughout. A return six months late owing £5,000 can carry well over £1,000 in penalties before interest. The £100 is not waived because your return showed nothing to pay, which surprises people every year. Reasonable excuse appeals do succeed for genuine causes — serious illness, bereavement, a service failure at HMRC’s end — but not for pressure of work.