Time limits may be varied by partiesE+W
2.11 Unless these Rules or a practice direction provide otherwise or the court orders otherwise, the time specified by a rule or by the court for a person to do any act may be varied by the written agreement of the parties.
(Rules 3.8 (sanctions have effect unless defaulting party obtains relief), 28.4 (variation of case management timetable—fast track) and 29.5 (variation of case management timetable—multi-track) provide for time limits that cannot be varied by agreement between the parties)