SA370 form to appeal against the penalty for late filing if you have a reasonable excuse for not filing your tax return on time. A reasonable excuse is something that prevented you from filing your return on time, for example, an unexpected illness or a death in the family.

It's important to note that, the penalty will not be automatically cancelled and you need to provide evidence of your reasonable excuse. you should also note that you will have to appeal within 30 days of the penalty notice if you want to appeal against the penalty.

It's recommended to contact HMRC as soon as you know you will not be able to meet the deadline, as they may be able to help you avoid a penalty altogether.