The best advice about deadlines is not to miss them in the first place.
This particularly applies to deadlines that affect the tribunal’s jurisdiction, such as lodging the ET1 or putting in an appeal. Missing these deadlines will cause very serious problems and is likely to derail the case entirely.
Missing case management deadlines, such as for exchange of documents or witness statements, is less immediately fatal, but can also cause problems. It is far better to meet them, unless it really is impossible to do so.
If a deadline has been missed, it is worth attempting to complete whatever the task was as soon as possible. It is much easier to explain to a tribunal why something was late than why it is still late.