Appeal deadlines in tax disputes are short. Missing one is easy. Recovering from it is not.
A late appeal remains possible. HMRC may agree to accept it where there was a reasonable excuse and the taxpayer acted promptly once that excuse ended. Failing that, the tax tribunal can grant permission. But permission is the exception, not the rule.
The tribunal starts from the view that statutory time limits exist for good reason. They promote finality. After a while both sides are entitled to treat the matter as settled, and documents, memories and witnesses all fade.
In deciding whether to let a late appeal progress, the tribunal broadly asks three things. How long and how serious was the delay? Why did it happen? And, weighing everything, including the prejudice to each side, what does fairness require? The apparent strength of the underlying appeal can play a part, but the tribunal will not usually hold a trial of the merits at this stage.
Thin explanations rarely succeed. Pressure of work, unopened post or assuming someone else was dealing with it will seldom carry weight, particularly where the delay is long.
Time limits are not technicalities. Treat them as the foundation of the case.