An intending appellant must file an appeal notice within 21 days after the date of the decision it wishes to appeal against (CPR 47.23(1) and 52.12(2)(b)). This is so notwithstanding that no permission to appeal is required. If a party has good reason for seeking a longer period in which to appeal it should apply for an extension of time, either on the occasion when the decision is made, or by including such an application in its appeal notice.
If a detailed assessment is carried out at more than one hearing the time for appeal will not start to run until the conclusion of the final hearing unless the court orders otherwise (CPR 47.14(7)).