From
BAladdy's 1 June post on the BA CityFlyer thread:
G-CERZ was operated for BACF from 30MAR14 for 14 months and was replaced by G-CDKA on 11MAY15. Both aircraft are expected to operate on the IOM and DUS routes for BACF until at least the start of S16 schedule.
That suggests G-CDKA would return to Eastern at the end of May 2016 unless the lease was extended or G-CDEB was available. I imagine BACF will make the appropriate commercial decision to carry on or not in due course. This would explain the 20 May 2016 end date in the current timetable.