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Send it within the deadline in your outcome letter, usually five working days.
The appeal should be heard by someone not involved in the original decision wherever possible.
A tribunal claim for unfair dismissal must normally be started within three months less one day of the dismissal, and appealing does not pause that clock.
I write the guides on HoursHQ and check every statutory figure against GOV.UK, Acas and the legislation itself before it goes live. Where the law is genuinely unclear, I say so rather than guessing.