Appealing late to the First-Tier Tax Tribunal needs to be very carefully worded and must reflect the case law that is commonly used in all the courts. Needless to say, it is very difficult to appeal late without a good excuse and there must be exceptional reasons.
The judgment of Lord Drummond Young in Advocate General for Scotland v General Commissioners for Aberdeen City  STC 1218 included a useful analysis of the way in which the judicial discretion to permit the making of late tax appeals ought to be exercised. That case was concerned with section 49 of the Taxes Management Act 1970, but is applied by the Tribunal to all tax appeals.
That court (in italics, below) found that a late tax appeal can only succeed if it follows one of the following 5 principals:
1. Is there a reasonable excuse for not observing the time limit, for example because the appellant was not aware and could not with reasonable diligence have become aware that there were grounds for an appeal?
This effectively means that a late appeal could be allowed if the taxpayer thought he would not succeed on appeal because of a lack of grounds, perhaps until he received professional advice.
2. If the delay is, in part, caused by the actions of HMRC, that could be a very significant factor in deciding that there is a reasonable excuse.
So, here it could be that a trader entered into communication with HMRC after an assessment was raised and HMRC failed to respond or failed to notify the taxpayer that entering into ongoing communication did not constitute an appeal. Often people think that, by arguing their point with HMRC, this stops the clock in effect and an appeal can be lodged once all attempts to resolve the tax issue have failed. This is not technical correct, but may constitute a reasonable excuse for asking for a late appeal to be allowed.
3. Once the excuse has ceased to operate, for example because the taxpayer became aware of the possibility of an appeal, did matters proceeded with reasonable expedition?
This shows why it is vital to appeal as quickly as you can once it is known that an appeal will be made out of time. You will need to account for every additional day of delay.
4. Is there prejudice to one or other party if a late appeal is allowed to proceed, or if it is refused?
In other words, does it make any real difference to HMRC if the appeal is allowed to be submitted late?
5. Are there considerations affecting the public interest if the appeal is allowed to proceed, or if permission is refused? The public interest may give rise to a number of issues. One is the policy of finality in litigation and other legal proceedings; matters have to be brought to a conclusion within a reasonable time, without the possibility of being reopened. That may be a reason for refusing leave to appeal where there has been a very long delay. A second issue is the effect that the instant proceedings might have on other legal proceedings that have been concluded in the past; if an appeal is allowed to proceed in one case, it may have implications for other cases that have long since been concluded. This is essentially the policy that underlies the proviso to s 33(2) of the Taxes Management Act. A third issue is the policy that is to be discerned in other provisions of the Taxes Acts; that policy has been enacted by Parliament, and it should be respected in any decision as to whether an appeal should be allowed to proceed late. Fifthly, has the delay affected the quality of the evidence that is available? In this connection, documents may have been lost, or witnesses may have forgotten the details of what happened many years before. If there is a serious deterioration in the availability of evidence, that has a significant impact on the quality of justice that is possible, and may of itself provide a reason for refusing leave to appeal late.
Many taxpayers come to CTM because insolvency action has been commenced by HMRC. Whilst attempting to appeal to the First-Tier Tax Tribunal (and then possibly taking that argument further to the Upper Tribunal) may delay HMRC debt recovery/insolvency action by up to 18 months, this is not in itself grounds for appealing out of time.